274 DR J. H. ASHW011TH AND DR JAMES RITCHIE ON THE 



gonophore "), except that the two ectoderm layers of the reduced umbrella have 

 become conjoined (text-fig. l). 



But a very different condition is presented by the sporosacs of Dicoryne corcferta 

 and D. conybearei (text-fig. 2), for in these the only layer of ectoderm is that in which 

 the reproductive products have been differentiated. This forms the external ectodermal 

 layer of the sporosac and secretes a thin cuticle over its free surface. The tentacle 

 or tentacles lie between this cuticle and the layer which formed it. The tentacles 

 therefore do not lie in the position of a " mesotheca," i.e. between an "ectotheca" 

 and " endotheca," as in a medusoid gonophore, and Allman's attempt to interpret 

 them as homologous with the canals of a medusa, which lie in the " mesotheca," 

 therefore falls to the ground. The tentacles have nothing to do with a " mesotheca " ; 

 there is, in fact, no " mesotheca " in the full-grown sporosac of Dicoryne, nor is there 

 any trace of it during the entire course of development of the sporosac. 



Further, the single ectodermic layer of the sporosac of Dicoryne does not agree 

 in mode of origin, and is clearly not homologous, with that covering the manubrium 

 of a medusiform gonophore, i.e. is not homologous with the " endotheca." If the 

 enveloping ectoderm of the sporosac can be regarded as a homologue of any layer 

 in a medusoid gonophore, it is with the external layer of the early stage of the 

 latter, which subsequently gives rise to the " ectotheca " and to the ex-umbrellar 

 ectoderm. 



Consequently the gonophore of Dicoryne cannot be regarded as a manubrium, 

 and we have already seen that the tentacles are not the homologues of radial canals. 

 It may be noted that the tentacles of the sporosacs of Dicoryne are solid, and not 

 " free tubes," as Allman stated. 



Several authors have followed the view propounded by Allman, that the sporosac 

 represents a degenerate medusa which has lost. its umbrella but retained two radial 

 canals. Hincks (1868 [1869], p. 106) considered that it consisted "of a sexual 

 polypite (manubrium) furnished with two tentacles, which represent the more usual 

 natatory organ, while simple ciliary action replaces the propulsive movement of the 

 swimming bell." Delage and Herouard (1901, p. 64) and Kemna (1908, p. 280) give 

 a general adherence to Allman's " ingenieuse conception," reserving final judgment 

 to such time as the development shall be completely known. 



Chun has suggested an alternative view. He regards the sporosac as represent- 

 ing a much-compressed medusa in which the sub-umbrellar cavity has become 

 entirely closed and filled with sexual products. Regarding the tentacles he offers no 

 suggestion. " Bei ihnen [the sporosacs] ist die Glockenhohle vollstiindig geschlossen 

 und von den Geschlechtsproducten erfullt. In der Nalie des aboralen Poles des 

 Medusoids entspringen zwei lange opponirte Tentakel, die ebenso wie der diinn- 

 wandige Glockenmantel mit Cilien besetzt sind" (Chun, 1896, p. 264). Both 

 Delage and Herouard (1901, p. 64) and Kemna (1908, p. 280) allege that Chun 

 regards the tentacles as true marginal tentacles, and on this account readily demolish 



