A. Hyatt — Larval Theory of the Origin of Tissue. 345 



dence hangs necessarily upon the probability that the somites 

 of the embryo of Amphioxus imply descent from a segmented 

 animal; whereas, if we are correct, exactly the opposite view 

 may be considered as the more probable ; and the very close 

 comparisons made by Semper between what he considers homo- 

 genous organs and parts in Vertebrata and Vermes can only be 

 considered as evidence of the production of homoplastic effects 

 by means of similar modes of growth and the similar habits 

 of elongated and necessarily bilateral animals. 



We have objected to the theory that the Yertebrata may be 

 considered as descended from a Coelenterate ancestor because 

 the actinostome probably arose independently and very late in 

 the phylogenetic history of the Hydrozoa, and undoubtedly 

 arose independently in the Porifera. A stomodeum as it ap- 

 pears 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 ex- 

 tending 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 

 homologue of the internal actinostome of the Actinozoa. 

 These facts and the late stage at which it arises in the Actino- 

 zoa, during the gulinula stage, shows 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 phylogenetic mean- 

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

 distinct modes of development of the anterior invaginations of 

 the Yertebrata 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. The 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 proba- 

 ble that the notochordal tube, and the lateral differentiations of 

 the archenteron may have had a similar homoplastic simplicity 

 of structure. 



The development in Ascidia of the notochordal cells and 

 muscle cells from the walls of the archenteron invite the sug- 

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

 lateral muscles might have arisen as entirely disconnected and 



