Larval Theory of the Orujui of Tissue. 207 



in tlie pliylogenetic history of the Ilydrozoa, and undoubtedly 

 arose independently in the Porifera. A storaodeuni as it 

 appears in the ascula stage or in a sycon or ascon may be a 

 single opening not due to invagination, merely an enlarged 

 pore or outlet. The cloaca of the more specialized sponges 

 is first an outgrowth of the peripheral parts which becomes 

 inheritable and causes the appearance of the ectoderm as a 

 lining layer extending to an indefinite depth into the interior. 

 A stomodeum, also, does not exist in most of the Hydrozoa 

 except in the primitive shape of an outgrowth, the hypostome, 

 which is the horaologue of the internal actinostome of the 

 Actinozoa. These facts and the late stage at which it arises 

 (in the Actinozoa during the gulinula stage) show us that, so 

 far as these types are concerned, it is an independent and 

 homoplastic organ in all of them. 



There are no exact comparisons between the embryos of 

 Ascidia and Amphioxus and those of the Invertebrata which 

 seem to include any stages later than the planula. Those 

 that have been traced between the mesoblastic somites indicate 

 homoplastic organs, and seem to have no pliylogenetic mean- 

 ing so far as the whole of the Vertebrata are concerned. The 

 distinct modes of development of the anterior invaginations 

 of the Vertebrata show that they had a different origin from 

 the anterior tube of the actinostome, and cannot be considered 

 homogenous with that organ in the Coelenterata. Tlie medul- 

 lary invagination is at first a stomodeum arising as a funnel 

 around the blastopore, and then spreads forward in the shape 

 of two folds, which subsequently form a tube, and it is pro- 

 bable that the notochordal tube and the lateral differentiations 

 of the archenteron may have had a similar homoplastic sim- 

 plicity of structure. 



The development in Ascidia of the notochordal cells and 

 muscle-cells from the walls of tlie archenteron invites the sug- 

 gestion that no true diverticula exist in this type. That the 

 lateral muscles might have arisen as entirely disconnected and 

 more primitive structural elements than the coelomata is 

 shown by Kowalevsky's work on Cassiopea already quoted 

 (Soc. Friends of Nat. Hist. &c. Moscow, pi. ii. figs. 10-13). 

 In this Hydrozoon portions of the archenteric walls grow out 

 and become directly converted into muscles, but no coelom is 

 formed. 



The notochord may have primitively originated as a tube, 

 but connexion with the hypophysis seems to be a necessary 

 condition of this theory ; and though this is highly probable, 

 it is not proven. The homoplastic origin of the notochord, 

 when explained in this way, agrees with the subsequent origin 



