Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/568

552 552 HYDROZOA confined to deep pits (fovese nervosae) from each of which & tentaculocyst arises (discovered in the Scyphomedusoe in dependently by Schafer and Glaus). With some exceptions, medusae provided with ocelli are destitute of vesiculi, which alone occur in the vesiculate Leptomedusce. Tentaculocysts Fig. 13. Fig. 14. FIG. 13. Tentaculocysts of medusa: (A, of Pelagia; B, of Charybdcea). a, the free tentacle hanging in the notch of the disc; 6, stalk; c, enteric anal continued into it; d, enlarged portion of the canal; e, concretions on endodennal cells; /, pigmented ectoderm ; g, lens. (From Gegenbaur.) TIG 14. Cells from the olfactory pits (foveas nervo&amp;gt;se) of Aurelia. (After Schafer.) characterize to the exclusion of the ocelli and vesiculi the Trachomedusce and NarcomediLsce among Hydromedusce and all the Scyphomedusce, except Lucernaria, where they are replaced by &quot; colleto-cystophors.&quot; The nervous system has only recently been correctly recognized in medusae, though seen by .Agassiz as long ago as 1849, and described both by Fritz Miiller and Haeckel in certain forms (Geryonidce} more recently (1860). It differs remarkably in the two great groups into which the Hydrozoa are divisible. In the Scyphomedusoe there is no continuous nerve-centre, but around and about each tentaculocyst nerve-fibres and cells are grouped in such a way as to divide the disc into zones of nerve supply corre sponding to the number of tentaculocysts (usually eight). TIG. 15. Scattered nerve ganglion cells, c, from the sub-umbrella of Aurelia aurita. (After Schafer.) Both the Hertwigs (Nerven-System der Meduscn, 1878) and Ei.ner (Die Medusen, 1879) entirely missed in their re searches the large nerve-fibres and prominent ganglion cells (fig. 15) which were discovered by Professor Schafer of &quot;University College, London (Phil. Trans., 1879), in the JScypJiomedusce. The writer can confirm Schafer s observa tion of the existence of such fibres and ganglion cells in the region of the circular muscular zone on the oral face of the disc of Aurelia, immediately beneath the flattened epithelium of the ectoderm. Professor Clans of Vienna has independently described (&quot; Polypen nnd Quallen der Adria,&quot; 187,8) similar uerve-cells and fibres in Chry- taora and Charybdcea. Professor Schafer failed to ascer tain satisfactorily the origin and termination of the fibres, which appear, however, to originate in superficial ecto- dermal cells (&quot;sense-epithelium&quot;) in the neighbourhood of the tentaculocysts and in the cells of those organs, and to terminate without any plexiform connexion with one another in the muscular fibres. Eimer has described very abundant and excessively fine fibres, often moniliform, which extend from epithelial cells in the neighbourhood of tentaculocysts and form a network traversing the gelatinous substance of the disc in every direction. This observation, though supported by the fact that such fibres are indi cated by the extended experimental investigation of Eimer and of Romanes (Eimer, Die Medusen; Romanes, Phil. Trans., 1876, et seq.), is not confirmed by other observers, and the fibres described are regarded as skeletal tissue. If Eimer s fibres do not exist, the muscular tissue of the medusae must be regarded as acting to a large extent inde pendently of nerve-control; and this is borne out by Claus a observation of the absence of sense-organs and nerve-fibres from the swimming-bells of the Siphonopliora (compound medusae). In the Hydromedusce the nerve ganglion cells are grouped in a continuous ring around the margin of the disc, separated horizontally into an inferior and superior portion by the insertion of the velum. The difference in the form of the nervous system has led Eimer to propose the names Cydoneura for the Hydromedusoe and Toponeura for the Scyphomedusce. Amongst the latter, however, Charybdcea, having a continuous velum like Hydromedusoe, has also a continuous nerve -ring. Comparison and Relations of Hydriform and Medusiform Persons. A simple shortening of the vertical axis, and a widening of the hypostome, with obliteration of the lumen (but not of the cells) of the endoderm over a considerable region of the disc thus produced, suffice to convert the hydra- form into the medusa-form. 1 This change of proportion made (fig. 16), the sense-organs of the medusiform person have to be added, and the change is complete. Thus it be comes clear that we have to deal with one fundamental form, appearing in a lower, fixed, nutritive phase and a higher, locomotor, generative phase in the two cases respectively. The phylogeny of the Hi/drozoa and the historical relation ship of the two phases (hydriform and medusiform) appears to be as follows. A two-cell-layered sac-like form, with mouth and with or without tentacles, was the common ancestor of Hydrozoa, Anthozoa, and Sponges. The particular form which the proximate ancestor of the Hydrozoa took (1 in fig. 16) is most nearly exhibited at the present day in Lucernaria and in the scyphistoma larva (hydra-tuba) of Discomedusas. It was a hemispherical cup-like polyp with tentacles in multiples of four, with four lobes to the wide enteric chamber. This polyp, after passing a portion of its life fixed by the aboral pole, loosened itself and swam fieely by the contractions of the circular muscular fibres of its hypostome (sub-umbrella), and developed its ovaria and spermaria on the inner walls of the enteric chamber. This ancestor possessed, like its descendants, a very marked power of multiplication, either by buds or by detached fragments of its body. Accordingly it acquired definitely the character of multiplying by bud-formation during the earlier period of its life ; each of the buds so formed completed in the course of time its growth into a free swimming person. We must suppose that the peculiarities of the two phases of development became more and more distinctly developed, the earlier budding phase exhibiting a more elongated form and simple enteric cavity (hydra-form), which subsequently 1 This relationship, demonstrated by the Hertwigs discovery of the endoderm layer of the medusa s disc, differs from that supposed to obtain by Professor Allman. He supposed the medusa s disc to represent the coalesced tentacles of a hydra-form, and cited the webbed tentacles of Laomedea flcxuosa in support of the identification, which had at the time very much to commend it.