62 



HYDROZOA 



confined to deep pits (fovese nervosae) from each of which 

 a tentaculocyst arises (discovered in the Scyphomedmoe in- 

 dependently by Schafer and Clans). With some exceptions, 

 medusae provided with ocelli are destitute of vesiculi, which 

 alone occur in the vesiculate Leptomedusce. Tentaculocysts 



Fig. 13. ' Fig. 14. 



Fio. 13. — Tentaculocysts of medusa (A, of Pelagia ; B, of Charyhdeed). 

 Uf the free tentacle hanging in the notch of the disc; &, stalk; c, enteric 

 canal continued into it; d, enlarged portion of the canal; e, concretions 

 on eiidodermal cells; /, pigmented ectoderm; g', lens. (From Gegenhaur.) 



Fig. 14. — Cells from the olfactory pits (fovese nervosse) of Aurelia^ (After Schafer.) 



characterize to the exclusion of the ocelli and vesiculi the 

 Trackomedusce and Narcomedusm among Hydromedibsae and 

 all the Scyphomedv^ce, except iMcernaria, where they are 

 replaced by " colleto-cystophors." 



The nervous system has only recently been correctly 

 recognized in medusse, 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 Scyphomedusce 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). 



Fig. 15.— Scattered neiTe ganglion cells, c, from the sub-umbrella of Aurelia 

 aurita. (After Schafer.) 



Both the Hertwigs {Nerven-System der Medusen, 1878) and 

 Eimer (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 

 University College, London {Phil. Trans., 1879), in the 

 ScyphomeduscB. 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 Claus of Vienna 

 has independently described (" Polypen und Quallen der 

 Adria," 1878) similar nerve-cells and fibres in Chry- 

 saora 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 ("sense-epithelium") 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; Eomanes, Phil. 

 Trans., 1876, et seq.), is not confirmed by other observers, 

 and the fibres described are regarded as skeletal tissue. If 

 Elmer's fibres do not exist, the muscular tissue of the 

 medusse must be regarded as acting to a large extent inde- 

 pendently of nerve-control ; and this is borne out by Claus's 

 observation of the absence of sense-organs and nerve-fibres 

 from the swimming-bells of the Siphonophora (compound 

 medusse). 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 Oycloneura for the Hi/dromeduscB and Toponeura 

 for the Scyphomedusoe. Amongst the latter, however, 

 Charyhdcea, having a continuous velum like Hydromediism, 

 has also a continuous nerve-ring. 



Comparison and Belations 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.'- 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 Hydrozoa 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, 

 Anthazoa, 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 Zucernaria 

 and in the scyphistoma larva (hydra-tuba) of Discomedusm. 

 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 freely 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 tliat 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. Hfe supposed the medusa's disc to 

 repre.sent the coalesced tentacles of a hydra-form, and cited the webbed 

 tentacles of Laomedea flexuosa in support of the identification, which 

 had at the time very much to commend it. 



