60 



HYDROZOA 



however, occur on the ectodermal cells, aad the solid ten- 

 tacles are essentially tactile organs. Placed in and between 

 the large cells of the ecto- 

 derm {Hydra, Cordylophora, 

 AUman, Kleinenberg, F. E. 

 Schulze) are small nucleated 

 cells which become con- 

 verted into vesicles contain- 

 ing a three-barbed (figs. 4 

 and 5) or simple filament 

 (nematocysts). These are 

 frequently grouped on the 

 surface in wart-like pro- 

 cesses or "batteries." Ne- 

 matocysts also are found in 

 the endoderm; but it is prob- 

 able that their presence 

 there is due to their having 

 been swallowed. 



The endoderm is usually 



but one cell deep, and lines Fig. S.— Nematocyst of Hydra, showing 



the entire cavity of the body ceii-mbstance and nucleus, cyst, tng- 



.i.i^^ ^^^^^^^,a, M.i,j ui uLio u^^j g^j. jjj^jj.^ ^^^ everted thread. (After 



startmg from the margin of f. e. sohuize.) 

 the mouth. In the region of the body proper, and in hollow 

 tentacles, the ceUs are ciliated (fig. 4). In this region they are 

 concerned in the secretion of digestive fluids and in absorp- 

 tion, and sometimes contain coloured granules (hepatic?). All- 

 man found in Myriotkela {Phil. Trans., 1875) that the endo- 

 derm cells project processes 

 like the pseudopodia oi Pro- 

 tozoa, and suggests that solid 

 food particles are incepted 

 by them. T. J. Parker has 

 published similar observa- 

 tion on Hydra (1880). In 

 the solid tentacles the en- 

 dodermal cells are greatly 

 modified, forming a kind 

 of skeletal tissue, each cell recalling by its vacuolation 

 and firm cell-wall the characters of vegetable parenchyma 

 (fig. 6). In the stems of Siplwnophora endoderm cells give 

 origin to muscular processes like those of the ectoderm 

 (Glaus). This latter fact has a morphological significance 

 which cannot be too gravely estimated. 



Generative products are not developed by any hydriform 

 persons (excepting the LucernaricB), the sexual process being 

 carried on by a distinct set of buds developed on the sides 

 of hydriform persons. These buds either become medusi- 

 form persons, or are degenerated representatives of such 

 persons (sporosacs) (figs. 17 and 18). Even the fresh- water 

 Hydra (fig. 42) does not appear to be an exception to this 

 generalization. The single egg-cell of Hydra projects at 

 the breeding season in an ectodermal covering, as a wart, 

 from the lower part of the body. A conical eminence or 

 two nearer the mouth contains the spermatozoa. Each 

 ovarium and each spermarium represents an aborted gene- 

 rative person. According to Kleinenberg the egg-cell and 

 the sperm-cells are both derived from the ectoderm. The 

 Lucemarice develop internal generative organs (fig. 19) 

 which correspond closely with those of the medusiform 

 persons of the group Scyphomedusce (see below), with which 

 they are classified. Both ova and testis are endodermal in 

 origin in Lucernaria and in the medusiform persons of the 

 Scyphomedusm, whilst they appear to be ectodermal in 

 origin in the complete medusiform persons of Hydro- 

 medusae, though in the degenerate medusiform persons 

 known as sporosacs they may either or both have an 

 endodermal origin. 



Medusifoem persons usually present themselves as 

 isolated free-swimming individuals, but like hydriform 



Fig. 6. — Vacuolated endodeim cells of carti- 

 laginous consistence from the axis of the 

 tentacle of a Medusa {Cunina). (Fi-om 

 Gegenbaur's Elements of Comparative 

 Anatomy.) 



persons they have the power of producing new persons by 

 budding (figs. 44, 45, and 46), which may become detached 

 or may remain connected with the primary person (fig. 57) 

 to form a freely swimming colony {Siphonophora) compar- 

 able to the fixed colonies of hydriform persons. Medusi- 

 form persons are often produced as the immediate result of 

 the development of the diblastula without any intermediate 

 hydriform phase {Pelagia among Scyphomedusce, Tracho- 

 medusoE, Narcomedusae, and probably sova^Anthomedusce and 

 Leptomedusce), but quite as frequently originate as lateral 

 huds upon the body-walls of hydriform persons (figs. 34, 

 37, and 43), or of other medusiform persons (see below), or 

 as metameric fission-products of hydra-forms. The typical 

 medusa-form is a hemispherical cup (the nectocalyx, or 

 umbrella, or disc), from the centre of which rises up a 

 cylindrical or conical process (the manubrium, erroneously 

 polypite) at the summit of which is the mouth (4, 5 in fig. 

 16). Four perradial (see above for use of this term) ten- 

 tacle-like lobes very commonly surround the mouth, or 

 numerous small tentacles (fig. 58), whilst the margin of 

 the disc is beset with tentacles four in number, or a mul- 

 tiple of four (sometimes six, or one only, or indefinite). 

 The aboral pole is dome-like, and is never attached except 

 in those forms which take their origin as buds on a hydri- 

 form colony when the connexion exists at this point. The 

 tentacles are, as in the hydriform persons, some solid, some 

 hollow : both occur in the same individual. 



Fig. 7. — Fortions of sections through the disc of medusae, — the upper one otUiiia, 

 the lower of Aurelia, el, endodei-m lamella, or vascular lamella; m, muscular 

 processes of the ectoderm cells in cross section ; d, ectodeim ; en, endodeim 

 lining the enteric cavity ; e, wandering endoderm cells of the gelatinous sub- 

 stance. (After Hertwig.) 



The body is not so completely hollowed out as in the 

 hydriform persons. The mouth leads into a straight tube 

 (the stomach) which occupies the axis of the manubrium, 

 and expands at its insertion into the disc. The disc, even 

 when thick and fleshy, is not fully excavated by the enteric 

 cavity. In young forms the cavity does occupy it right up 

 to the margin, but gradually the lumen disappears (fig. 29), 

 leaving a series of canals and a continuous plate of endo- 

 derm (fig. 7) formed by the coalesced walls of the space (the 

 endoderm-lamella of the Hertwigs, see Organismus der 

 Medusen, 1878 ; the vascular-lamella of Glaus, " Polypen 

 und Quallen der Adria," Wiener Denhek., 1878). The 

 peripheral portion of the lumen of the original enteric cavity 

 forms the ring-canal, which runs all round the margin of 

 the disc, and is continued into the hollow tentacles. The 

 lumen is further retained at intervals in the form of radiat- 

 ing canals connecting the axial enteric cavity with the ring- 

 canal. These may be perradial, interradial, and adradial 

 (see above as to tentacles of hydra-form), and may branch 

 dichotomously in the disc or form networks. 



The medusae are thicker and more fleshy to the touch 

 than are the hydra-forms, and are at the same time trans- 

 parent. This is entirely due to the enormous development 

 of a structureless substance between ectoderm and endoderm, 

 corresponding to the " Stutz-lamella" or structureless lamella 

 of the hydra-forms. (See figs. 49 and 51, representing 

 sections of Carmarina and of Cunina.) 



