1884.] " 113 [Hyatt. 



of the endoblast due to the earlier inheritance of the tendency 

 to extra growth on the part of the periphery. We have by no 

 means intended in the above remarks to indorse the opinion that 

 the coelomic sacs and tubes, or the neural canal and mesenteron 

 of the Vertebrata were necessarily derived from some preexisting 

 ancestor among the Coelenterata. On the contrary we have used 

 these comparisons between different but, as Cope would would call 

 them, homologous forms (Origin of Genera, Proc. Acad. Sci. 

 Philad., 1868, p. 53) to show that the Porifera, Coelenterata and 

 possibly Vertebrata were derived from some common undis. 

 covered form, possibly a gastrula, as parallel types and not as nec- 

 essarily homogenous and diverging from a common, but neverthe- 

 less coelenterate ancestor, as seems to be Sedgwick's view. The 

 comparisons made between Amphioxus and the Actinozoa are 

 open also to the very serious objection, that, as shown below 

 the coelomic sacs in the Actinozoa and Hydrozoa and Porifera 

 are purely homoplastic organs arising from different structural 

 modifications, from buds of the archenteron in Porifera and from 

 vertical ingrowths of the endoderm in the Hydrozoa and Actino- 

 zoa. Thus the surroundings may be supposed to have pro- 

 duced a similar effect upon Amphioxus and theAscidia; and 

 the coelomic sacs and somites of the vertebrates, though strictly 

 homologous derivatives of the archenteron, can be considered as 

 entirely independent in origin from those of the Actinozoa, and 

 possibly all other types. 



The characteristics of the diverticula show that in the spon- 

 ges these organs have a peculiar, and often dendritic distri- 

 bution and primitive bag-like form, which afford strong con- 

 trasts with the same parts as they appear in the Hydrozoa and 

 Actinozoa. Anatomically the sponges may be called Metazoa 

 protocoelomata, and by the use of this term can be distinguished 

 from the Radiata, or Metazoa trochocoelomata (animals having 

 vertical and radiatory coelomata), and from the paired or yoked 

 coelomates in which the metamera arise, as in Peripatus and 

 Amphioxus, from bilateral diverticula. We can readily trans- 

 form a protocoelomate into a trochocoelomate by destroying the 

 horizontal parts of the partitions between the ampullae in a 

 Sycones so as to resolve those sacs which lie immediately above 

 each other in the same line into single cavities. This would 



PnOCKEDIKGS B. R. X. H. VOL. XXIII. 8 APRIL., 1885. 



