DEVELOPMENT OF CHOK DA- ANIMALS. 85 



A^e will consider as the seventh stage in the human 

 pedigree. 



An examination of the Comparative Anatomy of the 

 various Scolecid forms, which we might perhaps distinguish 

 here, would lead us much too far into the difficult details 

 of the Comparative Anatomy and Ontogeny of the Worms. 

 For our purpose it seems more important to call attention 

 to those phylogenetic advances, by means of which the 

 organization of the earliest Blood-bearing Worms was in 

 the end elevated to that of the Chorda-animals. The Com- 

 parative Anatomy and Ontogeny of the Gliding-worms 

 and of the Ascidians justify us in giving special weight to 

 the significant differentiation of the intestinal canal into two 

 distinct divisions ; into an anterior division (the gill-intes- 

 tine), which accomplishes respiration, and a posterior divi- 

 sion (the stomach-intestine), which accomplishes digestion. 

 As in Gastrseads and Primitive Worms, so also in the Ascidian 

 larva, the intestinal canal is at first a simple pouch-like 

 body, provided merely with a mouth-opening. A second 

 opening, the anus, does not develop till a later period. Gill- 

 openings afterwards appear in the anterior section of the 

 intestinal canal, by which the whole anterior intestine is 

 transformed into a gill-body. This remarkable arrange- 

 ment is, as we found, quite peculiar to Vertebrates, and, 

 except in the Ascidians, occurs nowhere else. Among extant 

 Worms there is, however, a single isolated and very remark- 

 able Worm form, which in this respect may be regarded 

 as distantly allied to the Ascidia and to Vertebrates, and 

 perhaps as an off-shoot from the Soft-worms (Scolecida). 

 This is the so-called " Acorn- worm " (Balanoglossus, Fig. 

 186), which lives in the sand of the sea-shore. The in- 



