Hyatt.] 122 [March 5, 



their original stock form. The Porifera in relation to the Coe- 

 lenterata have been generally considered degenerate, and yet 

 there are strong reasons against this opinion ; and the Ascidia 

 seem to us to bear a somewhat similar relation to the Vertebrata. 

 Their embryonic history has no stage which exhibits, as does 

 that of Cirripedia and many parasites, a distinct type-larval stage 

 by which we can definitely show that they are degraded forms of 

 a vertebrate type higher than Amphioxus. If the Ascidia are 

 admitted to be Vertebrata, it is evident that they must have 

 sprung from some lower type than the Amphioxus, and one in 

 which the formation of a notochord was in a still more immature 

 condition. The tail and its four muscles, though hard to account 

 for, unless the Ascidian is a vertebrate, are the highest char- 

 acteristics shown by the embryo ; and these do not bespeak any 

 origin more exalted than that of some type considerably less dif- 

 ferentiated than Amphioxus. 



The formation of the lateral muscles from lines of cells derived 

 from the lateral walls of the archenteron, as shown by Kowalevsky, 

 is a case of direct conversion, and it remains to be determined 

 whether these myoblasts can be considered as representing 

 archenteric diverticula. Similar conversions occur in Cassiopea 

 (Kowalevsky, Soc. Friends Nat. Sci., vol. x, pt. 2, pi. 2, f. 8-10), 

 and yet these myoblasts are not diverticula, nor are the muscles 

 so considered in other types when derived from the endoderm. 

 Dohrn (ITrsp. d. wirb. Thiere, Leipz. 1875) supported the theory 

 of defeneration with strong arguments, but these are taken from 

 comparisons of the later stages of growth in Ascidia with the 

 adults of Cyclostomata ; and, if there are fundamental differences 

 in the embryo, as suggested by Kowalevsky, they serve to show 

 the affinites of Ascidia with vertebrates, but not that they are 

 degenerate descendants of Cyclostomata, or even of Amphioxus. 

 Dohrn's law of the development of latent functions can be applied 

 whether the changes are degenerative or progressive. It seems 

 to fully account for the transmutations of structure which occur 

 in the Ascidia, but this j>oint is not essential to the present dis- 

 cussion. 



We may assume, if we choose, that the Ascidia are degene- 

 rate as regards some free moving ancestry, but the Porif- 

 era are degenerate in this respect, and so are all attached ani- 



