genera a varying amount of degeneration and atrophy, of which we have in existence all possible degrees, leading from the fixed "phanerocodonic gonophors" (Allman, bell-like genital buds) of many Siphonophora through C D E develop their genitalia from the endoderm, and are (to use Rapp's terms) Endoari whilst the former are Exoari. But the bodies mistaken for external generative organs by Rapp and other early observers in many hydroids, and in Hydra itself, are aborted degenerate medusæ. (4.) A further set of changes, which have affected the original hydriform colonies and their medusa-buds so as to produce new complications of structure among the Hydromeduse, are summed up under the head of polymorphism." The differentiation of hydriform and medusiform persons is a case of dimorphism; a further distribution of functions, with corresponding modification of form, gives us "polymorphism." PolymorFIG. 17.-Diagrams illustrating the gradual degeneration of the medusa- phism is unknown in the Scyphomedusa, and it is bud into the form of a sporosac. The black represents the enteric chiefly confined to two groups of Hydromeduse cavity and its continuations; the lighter shading represents the (the Hydrocorallina and the Siphonophora). genital products (ova or sperm). A, medusiform person still attached by a stalk at the aboral pole to a colony (phanerocodonic gonophor In the hydriform colonies of Hydractinia (one of Allman); B, modified medusiform person, with margin of the disk of the Gymnoblastea-Anthomedusa) the outer (umbrella) united above and imperforate (mouthless) manubrium (adelocodonic gonophor of Allman): C, sporosae, with incomplete ex- hydriform persons of the colony (Fig. 39) differ in tension of the enteric cavity into the umbrella, rudimentary form from the rest, and have wart-like tentacles. invagination above to form the sub-umbrella cavity: D, sporosae with In the same genus, and also in many Calyptoblastea, manubrial portion only of the enteric cavity; E, sporosac without any trace of manubrium. FIG. 18.-Two female sporosacs (degenerate medusa) of Hydractinia echinata. (From Gegenbaur, after Van Beneden.) a, ectoderm; b, endoderm; o, egg-cells; g, enteric cavity. In A an invagination of the ectoderm, which is more complete in B, represents the rudiment of the sub-umbrella space. the hydriform persons which are destined especially to give origin to medusa-buds are devoid of tentacles and mouth, and are known as blastostyles (Allman), (Fig. 43). In Hydrocorallince (Fig. 53) elongated hydriform persons (dactylozooids) with no mouth and sporadic tentacles are set in series around a central short mouth-bearing person (gastrozooids) forming the "cyclo-systems" of Mr. Moseley (Figs. 52 and 55). In the Siphonophora, in addition to nutritive (hydriform) persons and generative (medusiform) persons, there may be rows of swimming-bells (medusæ devoid of mouth and of genitalia), coveringpieces (flattened medusæ), and tentacle-bearers (hydriform persons with one long highly developed. tentacle) (see Figs. 56 and 57). Hypothesis of the Individuation of Organs.-The building up of complex individualities, such as a hydrozoon colony, a flowering plant, or a segmented worm or arthropod,-in any one of which a number of common units are repeated, but with varied form and function in each part of the compound body,―is generally admitted to be explicable in two ways, and which of the two explanations may be adopted in any one case must depend on the ultimate inference from a wide series of observations. The first hypothesis, which undoubtedly applies to the ordinary hydriform colonies of Hydrozoa, to the segments of Taenia, and to plants number of Hydromedusa (Figs. 35, 38, 39, 40, and 42) formed by the repetition of phyllomes, is that an have lost all evidence of the real characters of their original unit like those which constitute the composite medusa-forms, just as others have suppressed the evi- organism has freely budded, and repeated its own dence of their hydra-forms by direct development from structure in the well-marked units which remain conthe egg; and inasmuch as both these processes take joined to form an aborescent or linear aggregate. place in genera, having the closest affinity with genera This is "eumerogenesis," and such aggregates may be in which both hydra-form and medusa-form are fully termed eumeristic. By a division of labor and consepreserved, it is not possible to erect groups similar to quent modification of form among the units of a eumerthe Haplomorpha of Carus or the Monopsea of All-istic aggregate, such an aggregate may (in the course man for their reception. The difficulty of classifica- of phylogeny) acquire varied shape and definite grouption is, however, rendered very great, for a double system becomes necessary, which shall deal with the characters of hydriform and medusiform persons in parallel equivalent series. The difficulty is considerably enhanced when we find that identical medusa-forms may spring from unlike hydra-forms, and, conversely, that closely allied hydra-forms may give rise to very different medusa-forms. The character first noticed by Rapp as distinguishing the hydroid polyps from the coral polyps, namely, that of developing their genitalia as external bodies (Exoari) instead of internally (Endoari), is seen by the considerations just adduced to be fallacious. The Hydromeduse, it is true, often (not always) develop their generative products from the ectoderm, and the genitalia frequently project as ridges and discharge themselves directly to the exterior in this division. The Hydromedusa contrast in this respect with the Scyphomedusa and Anthozoa, which ing of its constituent units, and a high specialization as an individual. The high degree of individuation which may be thus attained is due to the more or less complete synthesis of a eumeristic colony. The more highly individuated Chatopods and Arthropods are synthesized linear colonies. The cyclo-systems of the Hydrocorallinae are undoubted examples of synthesized colonies. The second hypothesis is one which is applicable to cases which, in the absence of special evidence to the contrary, might be regarded as highly synthesized colonies. According to this second hypothesis, such highly individuated composite organisms have not (in their phylogeny) passed through a eumeristic phase in which the units were well developed and alike, but the tendency to bud-formation (whether lateral, linear, or radial) has all along acted concurrently with a powerful synthetic tendency, so that new units have from the first made but a gradual and dis Definition of the Hydrozoa.-The Hydrozoa are Colentera nematophora, distinguished from the fellowgroup Anthozoa (the name applied to Actinozoa when the Ctenophora are removed from them) by not possessing the latter's constant and sharp differentiation of the archenteric cavity into axial digestive and periaxial septate portions, usually by a simpler form of nematocyst, and generally by lower histological differentiation.' The following is a brief summary of the chief characters of the larger divisions of the Hydrozoa: Sub-class I. ScYPHOMEDUSA.-These are Hydrozoa which in the adult condition always have four or eight guised appearance. This is "dysmerogenesis," and a synthesized eumeromorph, but may be a dysmerosuch aggregates as exhibit it may be called dysmeristic. morph. In dysmeristic forms the individuality of the primary unit dominates from the first, and the merogenesis (segmentation or bud-formation) can only show itself by partially here and more completely there compelling (as it were) the organs or regions of the body of the primary unit to assume the form of new units. The arms of star-fishes are, when we consider them as derived from the antimera of a Holothurian, explained as examples of dysmerogenesis. So, too, the series of segments constituting a leech, and probably also the segments of a vertebrate. Eumerogenesis and dysmerogenesis are only variations of one process, merogenesis, and no sharp line can be drawn between them. Individuation may appear at any period in the phylogeny of a eumeristic aggregate and synthesize its units. On the other hand, individuation is more or less completely dominant throughout the history of a dysmeristic aggregate, and is gradually broken down as a more and more complete analysis of the primary unit into new units is effected. It will be observed, however, that in dysmerogenesis, the form which individuation tends to preserve is that of the primary unit (notably the case in leeches as compared with the ameristic flukes), whereas when we have eumerogenesis followed by synthesis, the resulting form-individuality is something absolutely new. Thus, using the terms eumeromorph and dysmeromorph, we have(1) synthesized eumeromorph simulates normal dysmeromorph; (2) analysized dysmeromorph simulates normal eumeromorph. FIG. 19.-Diagrammatic vertical section of a Lucernaria in the plane of an interradius. a, one of the interradial angles of the disk, giving rise at a' to two groups of tentacles adradial in position; b, axial enteric cavity; e, endoderm; d, band-like genital gland (ovary or testis), adradial in position, and attached to the interradial septum which runs along the angular process of the disk, to which the letters c, d point; p, aboral region or "foot" z, the interradial gastral filaments or phacella. (After Allman.) interradial groups of gastral filaments ("phacella" of Haeckel) (Figs. 16 (6), 23, and 26). The genitalia (ovaria and spermaria) are developed from endoderm, and are always interradial (in the four radii formed after the first four). The hydra-form is not a "hydroid," but a short polyp with broad hypostome,the "scyphistoma," which gives rise to medusa-forms by transverse fission (strobilation), or itself develops genitalia (Lucernaria). Combined visual and auditory organs in the form of modified tentacles (tentaculocysts) to the number of four, eight, or more occur on the edge of the disk (except in Lucernaria, where they are represented by the "colleto-cystophors"). The medusa-form in some cases develops from the egg without the intermediate scyphistoma-stage (Pelagia, Charybdaa?). The edge of its disk is provided with lappets, which cover the sensorial tentaculocysts (hence Steganophthalmia of Forbes), and is not provided with a velum (hence " Acraspeda" of Gegenbaur), excepting the rudimentary velum of Aurelia (Fig. 31) and the well-developed vascular velum (pseudo-velum) of Charybdaa (Fig. 21). There is no continuous marginal nerve-ring (except in Charybdaa), but several separate marginal nerve-centres (hence Toponeura of Eimer). The diblastula in all cases, as yet observed, Whether the fixed hydriform colonies of the Hydrozoa, with their more or less complete medusiform buds, and further, the floating colonies of Siphonophora, with their polymorphous units, are to be regarded as synthesized eumeromorphs or as dysmeromorphs, more or less analyzed, is perhaps still open to discussion. The former view (that adopted here) is that held by Allman (Monograph of the Tubularian Hydroids, 1874), by Leuckart (1851), by Gegenbaur (Grundriss, 1874), by Claus (Grundzüge der Zoologie, 1876), and by the Hertwigs (Organismus der Medusen, 1878). On the other hand, Huxley (Oceanic Hydrozoa, 1856), formerly Gegenbaur (Zur Lehre der Generations-Wechsel, 1854), and, more recently, Ed. Van Beneden ("De la distinction originelle du testicule et de l'ovaire," Bull. Acad. Roy. Belg., 1874) have held that the medusiform person is a generative wart which has gradually assumed the characters of a bud, and that the various phases presented by it in different genera are so many more or less successful strivings after complete assumption of the hydra-form (from which the medusa-form is thus secondarily derived). Similarly the variously modified units of the siphonophorous colony have been regarded as the organs of a parent unit which have each more or less completely acquired the form of that parent unit, or, in other words, the colonies in question have been held to be dysmeromorphs. Recently ascertained facts as to the polymorphism of Hydrocoralling, but more especially the demonstration of the identity of structure of the meduse of the Scyphomedusan and Hydromedusan groups, and, further, the mode of development of the Scyphomedusa from the scyphistoma, and the rela-series, 1879) have insisted that in the Hydromeduse the genitalia tions of the generative products to the enteric cavity, combine to render the view that the polymorphous and dimorphous colonies of Hydrozoa are synthesized eumeromorphs more probable, in the judgment of the present writer, than that which would explain them as dysmeromorphs. The term merogenesis," and its subordinate terms, "eumerogenesis, dysmerogenesis," etc., are applicable to units of the first order, namely, cells, as well as to the "persons " which are built up by them. Ordinary cell-division is an example of eumerogenesis; freeformation of nuclei, as in the fertilized ovum of Arthropods, is dysmerogenesis. A syncytium is usually 1 Quite recently the Hertwigs (Jenaische Zeitschr., bd. vi., new (both ova and testes) are developed from the ectoderm, whilst in the Scyphomedusa and in the Anthozoa they develop from the endoderm. On this account they propose to abandon the grouping into Hydrozoa and Anthozoa of Calentera nematophora, and suggest two groups, the Ectocarpea and the Endocarpex, the former equiv alent to Hydromedusa, the latter embracing Scyphomedusa and Anthozoa. The Anthozoa exhibit a further predominance of the endoderm in its extensive origination in them of muscular fibre, which but rarely and in small quantity develops from endoderm their generalization on their own studies of meduse, but they in the Hydromedusa or in the Scyphomeduse. The Hertwigs base have ignored the observations of Van Beneden on Hydractinia and of Ciamician on various Tubularians, in which the origin of Fraipont has repeated an observation of Van Beneden's on Cameither sperm or ova from endoderm is established. Recently panularia, and shown conclusively that the ova in that form arise from endoderm. Weismann (Zoologischer Anzeiger, May, 1880) shows the same for Plumularida and Sertularida; the reader is referred to his paper. points to the cavity uniting neighboring pouches near the margin of the umbrella and giving origin to TCa, the tentacular canal; Ve, velum; Fr, frænum of the velum; Tc, tentaculocyst. FIG. 23. These are interradial. FIG. 20.-Charybdæa marsupialis (natural size, after Claus). The four an- FIG. 23.-Vertical sections of Charybdaa marsupialis, to the left in the plane The binary division of the Hydrozoa was established by Eschscholtz (System der Acalephen, 1829). whose Discophora phanerocarpa correspond to the Scyphomedusa, whilst his Discophore cryptocarpæ represent the Hydromedusa. The terms point to distinctions which are not valid. In 1853 Kolliker used the term Discophora for the Scyphomeduse alone, an illegitimate limitation of the term which was followed by Louis Agassiz in 1860. Nicholson has used the term in the reverse sense for a heterogeneous assemblage of those medusæ not classified by Huxley as Lucernaridæ, nor as yet recognized as derived from hydroid trophosomes. This use of the term adds to the existing confusion, and renders its abandonment necessary. The term Discomedusa was used for the Scyphomeduse by Haeckel in his Generelle Morphologie (excluding Charybdæa), whilst Carus (Handbuch, 1867) confines the term Medusa" to them alone, which is objectionable, since it belongs as justly to the Hydromeduse. Forbes's term for them, Steganophthalmia, indicates a true characteristic, failing only in the Lucernariæ, but its complementary term Gymnophthalmia is inaccurate. Similarly the terms Acraspeda and its complement Craspedota are unacceptable. Eimer has proposed to use the terms Toponeura and Cycloneura for the two divisions,but Charybdæa appears to break down this division as so many others. The old term Acalephæ, which is retained by Gegenbaur in its proper sense for all the Caelentera nematophora, is used as the designation of the Scyphomedusa alone by Claus (Grundzüge der Zool., 1878), which cannot fail to produce confusion. The term Lucernaridæ, proposed so long ago as 1856 by Huxley (Med. Times and Gazette), most truly indicates the relationships of these organisms which he was the first to recognize, but it seems desirable to restrict this term to the limited order in which Lucernaria is placed, and to employ for the larger group--Scyphomedusa a term which is the true complement of the convenient name assigned to the other division of Hydrozoa, viz., Hydromedusa.1 Order 1. Lucernaria,-Scyphomedusa devoid of tentaculocysts, with the aboral pole of the body produced into an adhesive disk by which the organism (which possesses the power of swimming by contraction of the circular muscular zone of the hypostome) usually affixes itself. The enteric cavity is divided into four perradial chambers by four delicate interradial septa. The genitalia are developed as four-paired ridges at the sides of the interradial septa on the oral wall of the chambers (Fig. 19). No reproduction by fission nor alternation of generations is known in the group. At the edges of the disk capitate tentacles are developed in eight adradial groups; between these are modified tentacles 66 some genera, the marginal anchors or colleto-cystophors. The canal system which has sometimes been described in them is a product of erroneous observation. A very few genera and species of this order are known. They may be justly called the coenotype of the medusa (James Clark), and their relationship to the free-swimming forms may be compared, as was done by L. Agassiz, to the relationship of the stalked Crinoids to such forms as Comatula. Three species are not uncommon on the British coasts. 1 Scyphomedusa (okúdos, a cup) are medusa which are related by strobilation to Scyphistoma,-a wide-mouthed polyp with four gastral ridges. Hydromedusa are meduse related to a Hydra-a narrower polyp, devoid of gastral ridges-by lateral gemmation. For use of these terms see paragraphs on Aurelia below. By Milne Edwards the animals forming this group were less common Rhizostoma of the Monogamelian Rhizostermed Podactinaria and associated with the Anthozoa. By toma, whilst Nausithoe and Discomedusa represent Leuckart they were termed Calycozoa; it is only of late the simple Cubostoma. that the closeness of their relationship to the Scyphomedusæ | Order 2. Discomedusa.-These are Scyphomedusa developing as sexual medusiform persons by transverse fission from a scyphistoma, or else directly from the egg. They have eight tentaculocysts, four perradial four interradial, and sometimes accessory ones (adradial). Four or eight genital lobes (ovaria or spermaria or hermaphrodite) are developed from the endoderm forming the oral floor of the central region of the enteric cavity, which is produced into a corresponding number of pouches. The mouth is either a simple opening at the termination of a rudimentary manu- The writer has adopted the term used by Haeckel for brium (sub-order Cubostoma), or it is provided with this order, and is indebted to his preliminary notices of a four or eight arm-like processes (sub-orders Semostoma large work on the Medusa, now in the press, for outlines of the classification and definitions which have been introand Rhizostoma). In the sub-order Rhizostome (Fig. duced with modifications in relation to these and the other 24 a), the edges of the oral opening fuse together at Medusa. The term Discophora is used by Claus (Grundzüge) an early age and leave several sucker-like secondary for the Discomedusa. It is quite clear from the varied and mouths, which were formerly mistaken for independent inconsistent use by different authors of that term, and also persons. The central enteric chamber is continued of the terms Acalephæ and Meduse, that they must be through the disk by a complicated, often reticulate, ejected altogether from use in systematic treatises. system of radiating canals, which excavate the endoderm lamella. FIG. 25.-Four stages in the development of Chrysaora. A, Diblastula stage; B, stage after closure of blastopore; C, fixed larva with commencing stomodaum or oral ingrowth; D, fixed larva with mouth, short tentacles, etc.; ep, ectoderm; hy, endoderm; st, stomodæum; m, mouth; bl, blastopore. (From Balfour, after Claus.) The structure of the common Aurelia aurita and its development have recently formed the subject of investigation by Claus, Eimer, and others. As the current accounts in text books are very inadequate, a short sketch of the morphology of that form is appended here. From the egg, according to the researches of Claus (whose figures, here reproduced, refer more especially to the closely allied genus Chrysaora, up to the completion of the scyphistoma), a single-cell-layered blastula develops which forms a diblastula by invagination (Fig. 25, A, B, C). The orifice of invagination closes up, and the ciliated "planula" (as this stage used to Le termed in all Colentera), after swimming around for a time, fixes itself, probably by the blastoporal pole. The true mouth then forms by inruption at the opposite pole. Two tentacles now grow out near the mouth opposite to one another (Fig. 25, D), and are followed by two more (Fig. 26), these indicating the four primary radii of the body which pass through the angles of the four-sided mouth, and are termed perradial. Meanwhile the aboral pole narrows and forms a distinct stalk, which in Chrysaora secretes a horny perisarc (Fig. 25, D). Four new tentacles, those of the intermediate or secondary radii, now appear between the first four, and are termed interradial. At the same time four longitudinal ridges grow forward on the wall of the enteric cavity (Fig. In the Semostoma and Rhizostoma (not in the 26). These interradial ridges have sometimes been Cubostoma) four remarkable (respiratory) sub-genital erroneously described as containing each a longitudinal pits (Fig. 28) are hollowed out in the gelatinous sub- canal connected with a circular canal at the base of the stance of the sub-umbrella (oral face of the umbrella). tentacles. They are in reality solid, as in the margin These do not communicate, as has been erroneously of the hypostome from which the tentacles spring. It supposed, with the genital organs, the products of is in connection with these four ridges that the gastral which normally are evacuated by the mouth. In the filaments will subsequently appear, as also the genital Tetragamelian Rhizostoma these pits remain distinct organs either along their middle line or adradially to from one another as in Semostomo, but in the Mono- them. The ridges correspond to the mesenteries of gamelian Rhizostoma they unite to form one con- the Anthozoa. Eight additional tentacles placed one tinuous sub-genital cavity placed between the wall of on each side of the perradial tentacles (or of the interthe enteric cavity and the polystomous oral disk. The radial, according as we may choose to regard the common English forms, Aurelia, Chrysaora, and matter) next appear, and are distinguished as adradial. Cyanaa, are types of the Semostoma, the somewhat All the tentacles reaching an equal size, we obtain the FIG. 24.-Scyphomedusa. a, Rhizostoma puimo; b, Chrysaora hyoscella. appearance seen in Fig. 26, when the young scyphis-four longitudinal gastral ridges further assimilate it. toma is looked at from above. Looked at from the The little creature is now about an eighth of an inch Tc GP 0 -- Tc FIG. 26. Later development of Chrysaora and Aurelia (after Claus). A, Scyphistoma of Chrysaora, with four perradial tentacles and horny basal perisarc. B, Oral surface of later stage of scyphistoma of Aurelia, with commencement of four interradial tentacles. The quadrangular mouth is seen in the centre; the outline of the stomach wall, seen by transparency around it, is nipped in four places interradially to form the four gastric ridges. C, Oral surface of a sixteen-tentacled scyphistoma of Aurelia. The four gastric interradial ridges are seen through the mouth. D, First constriction of the Aurelia scyphistoma to form the pile of ephyræ or young meduse (see Fig. 27). The single ephyra carries the sixteen scyphistoma tentacles, which will atrophy and disappear. The four longitudinal gastric ridges are seen by transparency. E, Young ephyra just liberated, showing the eight bifurcate arms of the disk and the interradial single gastral filaments. F, Ephyra developing into a medusa by the growth of the adradial regions. The gastral filaments have increased to three in each of the four sets. A, margin of the mouth; Ad, adradial radius; F, gastral filament; In, interradial radius; JG, adradial gastral canal; JR R3, adradial lobe of the disk; K, lappet of a perradial arm; M, stomach wall; Mst, muscle of the gastral ridge; Mw, gastral ridge; Ms, mesoderm; 0, tentaculocyst; P, perradial radius; R3, interradial radius; R, adradial radius; SG, commencement of lateral vessel. side, with its wide hypostome and short vertical axis, the scyphistoma differs widely from an ordinary hydra FIG. 28. Surface view of the sub-umbrella or oral aspect of Aurelia aurita, to show the position of the openings of the sub-genital pits, GP. In the centre is the mouth, with four perradial arms corresponding to its angles (compare Fig. 26). The four subgenital pits are seen to be interradial. x indicates the outline of the roof (aboral limit) of a sub-genital pit; y, the outline of its floor or oral limit, in which is the opening (compare 6 of Fig. 16). in height; in other genera, but not in Chrysaora, it ཨ་ wwww FIG. 29.-Half of the lower surface of Aurelia aurita. The transparent tissues allow the enteric cavities and canals to be seen through them. a, marginal lappets hiding tentaculocysts; b, oral arms; r, axial or gastric portion of the enteric cavity; gr, radiating and anastomosing canals of the enteric system; or, ovaries. The gastral filaments near to these are not drawn. (From Gegenbaur.) may now multiply by the production of a few buds from its fixed basal disk. After nourishing itself for a FIG. 27.-Development of Aurelia. Above to left, young scyphis- period, and increasing to four or five times the size toma with four perradial tentacles. Below to left, scyphistoma just noted, the vertical axis elongates and a series of with sixteen tentacles and first constriction. To the right, stro-transverse constrictions appear on the surface, marking bila condition of the scyphistoma, consisting of thirteen meta- off the body of the scyphistoma into a series of disks meric segments; the uppermost still possesses the sixteen tentacles of the scyphistoma; the remainder have no tentacles, (Figs. 26 and 27), each of which by the development but are ephyræ, each with eight bifid arms (processes of the of tentacles and completion of the constriction will bedisk). Each segment when detached becomes an ephyra, such as that drawn in Fig. 26, E, F. (From Gegenbaur.) come a separate medusa (in its young state called "ephyra"). The tentacles of the Aurelia and the form, and approaches the medusa-form, to which its structure of the margin of its hypostome are very dif |