HYDROZOA 



65 



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 con- 

 currently with a powerful synthetic tendency, so that new 

 units have from the first made but a gradual and disguised 

 appearance. This is "dysmerogenesis," and such aggregates 

 as exhibit it may be called dysmeristic. In dysmeristic 

 forms the individuality of the primary unit dominates from 

 the first, and the merogenesis (segmentation or bud-forma- 

 tion) can only show itself by partially here and more com- 

 pletely 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. Eutnerogenesis and dysmero- 

 genesis 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, in- 

 dividuation 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 dysmerogeuesis, tiie/orm which individua- 

 tion 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 dysmero- 

 morph, we have (1) synthesized eumeromorph simulates 

 normal dysmeromorph ; (2) aualysized dysmeromorph 

 simulates normal eumeromorph. 



Whether the fixed hydriform colonies of the Hydrozoa, 

 with their more or less complete medusiform buds, and 

 further, the floating colonies of Siphrmophora, with their 

 polymorphous units, are to be regarded as synthesized 

 eumeroinorphs or as dysmeromorphs, more or less analysed, 

 is perhaps still open to discussion. The former view (that 

 adopted here) is that held by Allrnan (Monograph of the 

 Tubtdarian Hydroids, 1874), by Leuckart (1851), by 

 Gegenbaur (Grundriss, 1874), by Glaus (Grundziige der 

 Zoolof/ie, 1876), and by the Hertwigs (Organismus der 

 Medusen, 1878). On the other hand, Huxley (Oceanic 

 Hydro-Mi, 1856), formerly Gegenbaur (Zur Lehre der Gene- 

 rations -IVechsel, 1854), and, more recently, Ed. Van Beneden 

 (" De la distinction originelle du testicule et de 1'ovaire," 

 Hull. Antd. Roy. Bely., 1874) have held that the medusi- 

 form 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 dysmero- 

 morphs. Recently ascertained facts as to the polymorphism 

 of HydrocoralliruB, but more especially the demonstration 

 of the identity of structure of the medusae of the Scypho- 

 medusan and Hydromedusan groups, and, further, the mode 

 of development of the Scypkomedusoe from the scyphistoma 

 and the relations 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," &c., 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; free-formation of 

 nuclei, as in the fertilized ovum of Arthropods, is dysmero- 

 genesis. A syncytium is usually a synthesized eumero- 

 morph, but may be a dysmeromorph. 



Definition of the Hydrozoa. The Hydrozoa are Ccelentera 

 nematophom, distinguished from the fellow-group Anthozua 

 (the name applied to Actinozoa when the Ctenophora are 

 removed from them) by not possessing the latter's constant 

 and sharp differentiation of the arch-enteric cavity into 

 axial digestive and periaxial septate portions, usually by a 

 simpler form of nematocyst, and generally by lower histo 

 logical differentiation. 1 



The following is a brief summary of the chief characters 

 of the larger divisions of the Hydrozoa: 



Sub-class I. SCYPHOMEDITS^E. These are Hydrozoa which 

 in the adult condition al- 

 ways have four or eight 

 interradial groups of 

 gastral filaments ("pha- 

 celle"ofHaeckel)(figs. 16 

 (6),23,and26). Thegeni- 

 talia (ovaria and sper- 

 maria) are developed from 

 endoderm, and are always 

 iuterradial (in the four 



radii formed after the first FlG . 19. Diagrammatic vertical section of a 



four). The hydra-form 

 is not a " hydroid," but a 

 short polyp with broad 

 hypostome the "scyphi- 

 stoma," which gives rise to 

 medusa-forms by trans- 

 verse fission (strobilation), 

 or itself develops genitalia 

 (Lucernarice). 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 

 disc (except in Lucerne/rice, where they are represented 

 by the "colleto-cystophors''), The medusa-form in some 

 cases develops from the egg without the intermediate 

 scyphistoma-stage (Pelagia, GJiarybdoea 1). The edge of 

 its disc is provided with lappets, which cover the sensorial 

 tentaculocysts (hence Steganophtluilmia of Forbes), and is 

 not provided with a velum (hence "Acraspeda" of Gegen- 

 baur), excepting the rudimentary velum of Amelia (fig. 31) 

 and the well-developed vascular velum (pseudo-velum) of 

 Charybdcea (fig. 21). There is no continuous marginal 

 nerve-ring (except in Charybdcea), but several separate 

 marginal nerve centres (hence Toponeura of Eimer). The 



1 Quite recently the Hertwigs (Jenaische Zeitschr., bd. vi., new 

 series, 1879) have insisted that in the ffydromedusas the genitalia 

 (both ova and testes) are developed from the ectoderm, whilst in the 

 Scyphomedusce and in the Anthozoa they develop from the endoderm. 

 On this account they propose to abandon the grouping into Hydrozoa 

 and Anthozoa of Ccelentera ncmatophora, and suggest two groups, the 

 Ectocarpece and the Endocarpece the former equivalent to Hydro- 

 medusce, the latter embracing Scyphomedusce and Anthozoa. The 

 Anthozoa exhibit a further predominance of the endoderm in its ex- 

 tensive origination in them of muscular fibre, which but rarely and in 

 small quantity develops from endoderm in the Hydromedvsa: or in the 

 Scyphomedusce. The Hertwigs base their generalization on their own 

 studies of medusas, but they have ignored the observations of Van 

 Beneden on Hydractinia and of Ciamician on vaiious Tubularians, in 

 which the origin of either sperm or ova from endoderm is established. 

 Recently Fraipont has repeated an observation of Van Beneden's on 

 Cainpanitlaria, and shown conclusively that the ova in that form arise 

 from endoderm. Weismann (Zoologischer Aiizcii/p/', May 1SSO) shows 

 the same for Plumularidce and Scrtulnrida:; the reader is referred to 

 his paper. 1 



Lucernai'ia in the plane of an intervuclius. 

 a, one of the inttM-radial angles of the 

 disc, giving rise at a' to two groups of 

 tentacles adradial in position; ft, axial en- 

 teric cavity; f, endoderm; d, band-like 

 genital gland (ovary or tcstis). adradial in 

 position, and attached to the inten-ndial 

 septum which runs along the angular pro- 

 cess of the disc, to which the letters c, d 

 point; p, aboral region or "foot"; z t the 

 interradial gastral filaments or phacelia?. 

 (After Allrnan.) 



