1884.] 123 [Hyatt 



mals from the nature of their habitat and habits. The original 

 stock forms of the different branches of the animal kingdom 

 probably possessed gradation and taxonomic rank, and we are 

 at liberty to infer that the Hydrozoa sprang as a division, or had 

 a starting point from animals of the original stock which were 

 taxonomically higher or more specialized than those from which 

 the Porifera were derived ; and possibly the Vertebrata may have 

 sprung from some type higher in zoological rank than the ances- 

 tor of the Coelenterata ; but we know very little about this orig- 

 nal stock, and can only infer from the embryos of animals that it 

 must have contained forms, as stated by Haeckel, in some way 

 similar to the gastrula and planula ; farther than this there 

 appears to be no solid ground for inference. Thus in the 

 Ascidia as worked out by Kowalevsky (Arch. Mikr. Anat. vol. vn, 

 1871) and other authors, there are, so far as we know, no suffi- 

 cient grounds for imagining that any ancestor of this division 

 possessed coelomic sacs or diverticula of the archenteron. The 

 cavity of the body in the adult is emphatically declared by 

 Kowalevsky to be continuous in its history with the primitive 

 blastocoel ; " bei den Ascidien die Leibeshohle doch aus der 

 Furchungshohle abstamme." It is evident, therefore, that no 

 argument for a coelenterate ancestor can be based on their struct- 

 ure in embryo or adult. Thus then, as the evidence now 

 stands, the coelomic sacs in Amphioxus must have been derived 

 from some higher type than Ascidia, — and probably, therefore, 

 were not present in the original ancestral vertebrate type, which 

 must have been more nearly related to Ascidia than it was to 

 Amphioxus. Balfour's idea that the Ascidia were more modified 

 in their development than the Cephalochorda, or Amphioxus, was 

 doubtless due to the fact that he compared the embryos with the 

 vertebrata rather than with in vertebrata ; whereas, according to 

 Kowalevsky, they have no coelom at any age, and in this respect 

 approximate the invertebrats more than any form of verte- 

 brates. They resemble, in their relations to higher forms of their 

 own branch, the Ascones among Porifera and the Intaeniolata 

 among Hydrozoa, which also have no coelomic sacs at any stage. 

 The difficulties included in the view that the planula and gas- 

 trula present separate and original modes of developing the endo- 

 derm is admirably put by Balfour, who shows that this implies 



