40 MORPHOLOGY. 



the umbrella, and like it may have a system of canals (/) more or less completely developed in 

 it, and may even present a rndimental codonostonie, while the most external layer («) (ectotheca) 

 corresponds to a similar external layer frequently present in the young medusa bud. 



It would seem that in no case is a velum or its homologue developed in the adelocodonic 

 gonophore, even though the representative of the umbrella should possess as in Tuhularia a 

 rudimental codonostome, while the marginal tentacles of the medusa are, except in the "me- 

 conidium " (see below, p. 57), also without their representative in this form of gonophore ; for 

 the tentacula-Iike tubercles which crown the summit of the sporosac of some of the Tuhdarkla 

 {Tuhularia larynx, for instance, PI. XXI) are of an entirely different significance, being merely 

 processes of the ectotheca. 



7. HoinoJo(/ical parallelism between Ilydra/dh and Medma. 



While we have been thus enabled to ti'ace a close parallelism between the medusa and the 

 sporosac, another comparison of great interest in this inquiry suggests itself, that, namely, 

 between the medusa and the hydranth. Now there can be little difficulty in finding in the 

 distal portion of the hydranth the homologue of the manubrium of the medusa ■} but the equivalents 

 of the umbrella and gastrovascular canals of the medusa are not at first sight so obvious. I 

 believe, nevertheless, that these are not totally lan-epresented in the hydranth. It will be kept in 

 mind that the tentacula of the hydranth are merely tubular radiating prolongations of the digestive 

 cavity, though with the cavity of the tube usually more or less obliterated by the peculiar condition 

 of the endoderm, and that for some distance from their origin they are necessarily included in the 

 thickness of the body-walls of the hydranth, where they consist merely of radiating canals extending 

 through these walls and lined by a layer of endoderm. Now this included portion I regard as the 

 true representative of the radiating cauals of the medusa ; and if we were to imagine the ectoderm 

 of the hydranth in a Eudendrium or Campanularia to acquire unusual thickness in a zone cor- 

 responding in position to the roots of the tentacles, we should have a disc-like extension of the 



^ Huxley (' Oceanic Hydrozoa') strongly insists on this relation, and is so impressed with the 

 closeness of the homology between manubrium and hydrautli, that he uses the same term, " polypite," 

 for both. 



Agassiz (op. cit., vol. iv, p. 326) has witnessed the very simple adelocodonic gonophore in male 

 specimens of his Rhizogeton fusiformis, instead of withering away after the discharge of its contents, 

 elongate itself, develop tentacles, and become transformed into a hydranth. I have myself, on one occa- 

 sion, seen an analogous phenomenon in the female gonophore of Cordylophora lacustris, in which, after 

 the discharge of the ova, the spadlx had become elongated through the ruptured chitinous investment of 

 the original gonophore, had developed an ectoderm, thrown out tentacles from its summit, and become 

 metamorphosed into an ordinary hydranth. In the case of Cordylophora the transformation is confined 

 to the spadix, while, according to Agassiz, the entire gonophore of Rhizogeton takes part in the meta- 

 morphosis. 



I believe that in both cases the phenomenon is an abnormal one ; it certainly is so in Cordijlo- 

 phora, for, in the ordinary conditions to which this hydroid is exposed, no metamorphosis of the kind 

 takes place. 



