SOFT-WORMS. 87 



and the Chorda-animals, yet, in virtue of this characteristic 

 structure of the gill-intestine, it may be considered a re- 

 motely allied collateral line of the Soft-worms. The 

 development of an anus (Fig. 186, a) at the end opposite 

 to the mouth, is also a considerable advance in the struc- 

 ture of the intestine. The further development of the 

 blood-vessel system in the Acorn-worm also indicates a 

 marked advance. In the ciliary surface of the skin, on 

 the contrary, it recalls the Gliding- worms. The sexes are 

 separated, while our scolecid ancestors were probably 

 hermaphrodite. 145 



From a branch of the Soft- worms, the group of Chorda- 

 animals (Chordonia), the common parent-group of the 

 Mantle-animals and Vertebrates also developed. The process 

 which primarily led to the development of this important 

 group of the ccelomati, was the formation of the inner 

 axial skeleton (the notochord, or chorda dorsalis), which 

 at the present day we find permanently retained in its 

 simplest form in the lowest Vertebrate, the Amphioxus. 

 We saw that this notochord is already found in the tailed 

 and free-swimming larva of the Ascidian (Plate X. Fig. 5). 

 The chorda does, indeed, serve specially as a support for 

 the rudder-like tail of the larval Ascidian, but its anterior 

 extremity passes in between the intestinal and medullary 

 tubes within the actual body of the larva. A transverse 

 section of this larva therefore shows that arrangement of 

 the most important organs which is characteristic of the 

 vertebrate type : in the centre is the firm notochord, which 

 supports the other organs and serves especially as a base 

 and point of attachment for the motive trunk muscles ; 

 above this notochord, on the dorsal side, is the central 



