HYDROZOA 



61 



The remarkable development of this substance in a hyaline con- 

 dition has led to the description of canals and spaces where none 

 exist — the supposed spaces being really occupied by this hyaline 

 substance. F. E. Schulze's statements as to extra-enteric spaces in 

 Sarsia are thus explained— and more decidedly the supposed circular 

 and longitudinal canals attributed by some authors to the scyphi- 

 stoma phase of Discomedusce. In the same manner (according to 

 Glaus) AUman's observations on Stephanoscyphus are reconciled 

 with those of F. E. Schulze on Spongicola — clearly the same form. 

 StepJiamoseypJms is devoid of either circular or longitudinal canals, 

 and though it has four remarkable ridges on the enteric wall like 

 those of the scyphistoma of ScyphoTmdusce (see fig. 26) stands in all 

 probability very close indeed to the Tubularian genus, Ferigonimus. 



Id a large number of medusa-forms the hyaline gelatinous 

 substance is structureless, but in many of the larger Scy- 

 phom^dwce it is occupied by in-wandering amoeboid cells de- 

 rived from the endoderm and by fibrous trabeculse (fig. 8). 



Fia. 8. — Gelatinous substance of the disc of Aurelia, showing— o, fibrous tra- 

 becule, and 6, wandering endoderm cells, with amoeboid movements. (From 

 Gegenbaur.) 



The wandering endodermal cells are nutrient in function, 

 and represent so far isolated elements of the enteric canal 

 system. 



The medusiform person is fundamentally adapted to 

 swimming movements. The muscular fibres are mostly 

 transversely striated, and are as a rule outgrowths of super- 



FiG. 9. — Muscular cells of medusffi (Uzzia). The uppermost is a purely muscular 

 cell fiom the sub-umbrella; the two lower are epldermo-muscular cells from 

 the base of a tentacle; the upstanding nucleated portion forms part of the 

 epidermal mosaic on the free surface of the body. (After Hertwig.) 



ficial ectoderm cells as in Hydra (fig. 9), (though in some 

 cases distinct cells) ; they are confined to a sheet spread on 

 the oral face only of the disc or swimming-bell (sometimes 

 called Sub-umbrella), to the extensile manubrium and 

 tentacles, and to an inwardly directed flap of the margin of 

 the disc known as the velum (Fe in 4 of fig. 16), which is 

 present in those medusae that are not flattened but conical 

 (bell-like), The muscular fibres on the oral face of the disc 

 and on the velum have a circular direction, interrupted 

 in some cases by radial tracts. The direction of the swim- 

 ming movements is obvious from this arrangement. 



The velum is not a constant element in the medusa's 

 disc ; it serves to contract the space by which water is 

 expelled from beneath the bell in the act of swimming. 



All fully-developed Hydromedusae possess the velum, but 

 only a few of the Scyphomedusw (Oharybdaia). In the 

 former the endoderm plate (vascular lamella) is not con- 

 tinued into it; in the latter vessels of the enteric system are 

 present in it (fig. 21), and, being probably morphologically 

 distinct, it has been here termed the "pseudo-velum." 



Unlike the hydra-forms, the medusa-forms of Hydrozoa 

 possess in addition to the tentacles highly-developed sense- 

 organs and ganglionic nerve-centres and nerves. The sense- 

 organs appear to be either eye-spots, or else otocysts, or 

 to combine the functions of both. In addition to these 

 are olfactory tracts or pits connected with the preceding. 

 The sense-organs are placed along the margin of the disc 

 (hence called marginal bodies), and are of three kinds: — 

 (1) ocelli — rounded pigment spots, rarely provided with a 



Fig. 10. Fig. 11. 



Fig, 10, — Ocellus of a medusa (LizziaKoeUikeri). oc, pigmented ectodermal cells; 



I, lens. (After Hertwig.) 

 Fig. 11.— Otocyst (formed entirely by ectoderm) of PMaltdium, one of the 



vesiculate medusse. d^, supei-ficial layer of ectoderm ; d^, deep layer of ecto- 



deiTn; A, auditory cells of ectoderm; AA, auditory hairs; np^ nerve body; 



Hri, upper nerve-ring ; r, endoderm cells of the circular canal. The otolith 



cavity is seen above A. 



lens (Lizzia) (fig. 10), always placed at the base of a tentacle 

 or in the radius of one on the oral surface (Lizzia), entirely 

 ectodermal in origin ; (2) vesiculi or otocysts — formed (as 

 discovered by the Hertwigs, 1878) by an invagination of the 

 ectoderm (fig. 11) containing concretions and hair cells ; 

 either open or entirely closed, generally numerous, and 

 placed between tentacles, sometimes at the bases of tentacles 

 (Obelia) ; (3) tentaculocysts — which are reduced and modi- 

 fied tentacles; into them alone of the three kinds of mar- 



FiG, 12. — Simple tentaculocyst of one of the TrachomedusoE (Rhopalonema 

 velaium). The process caiTying the otolith or concretion hk, foi-med by 

 endoderm cells, is enclosed by an upgi-owth forming the "vesk-le," which is 

 not yet quite closed in at the top. (After Hertwig.) 



ginal bodies do the endoderm and, in the more complex, 

 the enteric canal system enter (figs. 12, 13, and 30). The 

 endodermal sac forms the axis of the tentaculocyst, its cells 

 secrete crystalline concretions, and it functions as an otocyst; 

 pigment spots, which may have cornea, lens, and retina 

 well developed, are formed sometimes to the number of 

 six {Charybdcea) on the ectoderm of the tentaculocyst (fig. 

 13). The olfactory sense-epithelium (fig. 14) is either dis- 

 tributed in a continuous band on the margin of the disc 

 {HydromeduscB, discovered here by the Hertwigs), or it is 



