276 DR J. B. ASHWORTH AND DR JAMES RITCHIE ON THE 



distal end of the young female sporosac is thinner than that present elsewhere, there 

 being no attempt at the formation of a Glockenkern," and although young male 

 sporosacs exhibit distally a thickened ectoderm, the increased thickness is due to the 

 presence there of the spermatogonia. 



To summarise the evidence regarding medusoid homologies of the sporosac of 

 Dicoryne. This view derives no support from the facts of development, for there 

 is not at any stage a " Glockenkern," and therefore there cannot be a manubrium,* 

 nor is there any trace of umbrella or canals. The single or paired tentacles of the 

 sporosac of Dicoryne do not arise at the free end, where marginal tentacles are formed 

 in medusae, but at the proximal end, and the tentacles are therefore not equivalent in 

 the two cases. There is no evidence that the sporosacs of Dicoryne have undergone 

 regression from the condition of medusas or medusoid gonophores. If . they have 

 undergone such regression, it is so far-reaching as to have completely suppressed all 

 trace of the ancestral condition and to have brought the tentacles into an anomalous 

 position. 



2. The Polyp- Homology. 



Goette has suggested the polyp-homology of the sporosac of Dicoryne in place of 

 the earlier medusoid homologies. Having traced the earlier stages of formation of 

 the female sporosacs of D. conferta (although he failed to obtain the tentacle-bearing 

 stage), Goette came to the conclusion that there could be no question of medusoid 

 likeness, and that the resemblance to a degenerate polyp was much more evident : 

 ' Von einer medusoiden Bildung kann also bei den Gonanthen von Dicoryne gar 

 nicht Rede sein ; und mit viel grosserem Recht kann mann behaupten, dass die 

 basalen Tentakel dieser Gonanthen deren Polypen-ahnlichkeit unzweideutig hervor- 

 treten lassen ; es fehlt nur noch der Mund, um ein Hydranthenkopfchen vollstandig 

 zu machen " (1907, p. 68). According to this view, the body of the sporosac is 

 equivalent to the mouthless hypostome of a hydranth, the tentacles are normal in 

 position, but the proximal portion of the hydranth is wanting. 



Of the various theories advanced, Goette's polyp-homology appears to agree 

 most closely with the actual condition of the sporosac. But owing to the essential 

 simplicity of the structure and development of a polyp, resemblances between any 

 two-layered bud and a polyp may be taken for granted, and on this account the polyp- 

 homology is as difficult to prove as it is to refute. 



Weismann (1883, p. 245) has suggested that the fact that hydranths never spring 

 from the heads of other hydranths militates against the possibility that blastostyle- 



* This statement refers only to the development of medusae in the Anthomedusae, Leptomedusse, and perhaps 

 the Siphonophora. An interesting comparison might be made between the sporosac of Dicoryne, regarded as a 

 manubrium, and the larva of certain Narcoinedusac in which the manubrium first appears and two tentacles arise 

 subsequently near its aboral end. There is a general external resemblance between the free-swimming, two-tentacled, 

 ciliated sporosac of Dicoryne conferta and the free-swimming, two-tentacled, ciliated larva of Myinopsis rnediterranea, 

 four days old. But in its mode of origin and its subsequent history this larva differs entirely from the sporosac 

 of Dicoryne, and the general resemblance in external form is not to be taken as evidence of homology, but is probably 

 ■in instance of "convergence." 



