BLOOD-BEARING WORMS. S^ 



important a part in all the higher animals as the receptacle 

 of the very extensive intestines. The formation of this 

 coelom, and of the blood-vessels developed in connection with 

 it, exercised a very great influence on the further evolution 

 of the animal organization. The most important result was, 

 that it allowed the conveyance of rich nutritive juices to 

 those parts of the body lying near the circumference, and 

 developing at a considerable distance from the intes- 

 tinal canal. The intimate correlation, or reciprocity of the 

 parts, necessarily occasioned, in direct connection with the 

 progressive development of the blood-vessel system, many 

 other important advances in the structure of the body of 

 the Blood-bearing Worms. 



Just as among the Acoelomi, so also among the Coelomati, 

 the pedigree of our race must have passed through a large 

 number of diverse ancestral stages. But among extant 

 Coelomati (which form but a very small fraction of this once 

 multiform group), there are but very few Worms which can 

 with certainty be regarded as nearly allied to the long- 

 extinct ancestors of Man. In this respect, but a single 

 class of Coelomati is really of prominent importance ; these 

 are the Mantled Animals (Tunicata), to which belong the 

 Ascidia already known to us. Our careful examination of 

 the structure and germ-history of the Ascidian and the 

 Amphioxus have shown the extreme importance of these 

 very interesting animal forms. (Cf. Chapters XIII. and 

 XIV.) That examination fully justifies us in asserting 

 that among the ancestors of the Vertebrates (and therefore 

 of Man) there was an unknown extinct coelomate species, 

 to which the nearest allied form among extant anii lals is 

 the Appendicularia (Fig. 187), of w^hich we have already 



