Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/571

555 HYDROZOA 555 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 &quot; dysmerogenesis,&quot; 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. Eumerogenesis 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 dysmerogenesis, the/or??? 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) analysized dysmeromorph simulates normal eumeromorph. Whether the fixed hydriform colonies of the Hydrozoa, with their more or less complete medusiforni 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 analysed, 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 (Grundzuge der Zoologie, 1876), and by the Hertwigs (Organismus der Medusen, 1873). On the other hand, Huxley (Oceanic Hydrozoa, 1856), formerly Gegenbaur (Zur Lehre der Gene rations -Wechsel, 1854), and, more recently, Ed. Van Beneden (&quot; De la distinction originelle du testicule et de 1 ovaire,&quot; Bull. Acad. Roy. Belg., 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 Hydrocorallina, but more especially the demonstration of the identity of structure of the medusas of the Scypho- inedusan and Hydromedusan groups, and, further, the mode of development of the Scyphomedusce 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 &quot;merogenesis,&quot; and its subordinate terms, &quot;eumerogenesis, dysmerogenesis,&quot; &amp;lt;fec., are applicable to units of the first order, namely, cells, as well as to the &quot;persons &quot; 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 nematopkora, distinguished from the fellow-group 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 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. SCYPHOMEDUS^E. These are Hydrozoa which in the adult condition al ways have four or eight interradial groups of gastral filaments (&quot;pha- cellae&quot; of Haeckel) (figs. 16 (6), 23, and 26). Thegeni- talia (ovaria and sper- maria) are developed from endoderm, and are always interradial (in the four radii formed after the first FIG. 10. Diagrammatic vertical section of a four). The hydra-form is not a &quot; hydroid,&quot; but a short polyp with broad hypostome the &quot;scyphi stoma, &quot;which gives rise to medusa-forms by trans verse fission (strobilation), or itself develops genitalia (Liicernarice). 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 Lucernarice, where they are represented by the &quot;colleto-cystophors&quot;). The medusa-form in some cases develops from the egg without the intermediate scyphistoma-stage (Pelagia, Charybd&a 1}. The edge of its disc is provided with lappets, which cover the sensorial tentaculocysts (hence Steganophthalmia of Forbes), and is not provided with a velum (hence &quot;Acraspeda&quot; of Gegen baur), excepting the rudimentary velum of Aurelia (fig. 31) and the well-developed vascular velum (pseudo-velurn) of Charybdcca (fig. 21). There is no continuous marginal nerve-ring (except in Ckaryldcea), 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 Hydromedusce 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 nematophora, and suggest two groups, the Ectocarpece and the Endocarpecc the former equivalent to Hydro- ineduscr, the latter embracing Scyphomedusce and Anthozoa. The Anthozoa exhibit a further predominance of the endodenu in its ex tensive origination in them of muscular fibre, which but rarely and in small quantity develops from endoderm in the Hydromeduscc or in the Scyphomedusce. The Hertwigs base their generalization on their own studies of medusae, but they have ignored the observations of Van Beneden on Hydractinia and of Ciamician on various 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 Campanularia, and shown conclusively that the ova in that form arise from endoderm. &quot;Weismann (Zoofogischer Anzeiyer, May 1880) shows the same for Plumularidce and Scrtularidce; the reader is referred to his paper. Lucernaria in the plane of an Interradlns. a, one of the interradial angles of the disc, giving rise at a to two groups of tentacles adradiiil in position; 6, axial en teric cavity; c, endoderm; d, band-like genital gland (ovary or testis). adradial in position, and attached to the interradial septum which runs along the angular pro cess of the disc, to which the letters c, d point; p, aboral region or &quot;foot&quot;; 2, the interradial gastral filaments or phacellfe. (After Allman.)