76 PRIMITIVE ANIMALS 



blood carrying a special type of corpuscle charged 

 with respiratory haemoglobin, a ventrally situated 

 contractile heart, and a spacious coelomic body- 

 c.avity. The pharynx is also pierced or nearly 

 pierced at some period of the animal's existence 

 by a series of paired gill-slits, which may thus put 

 the cavity of the pharynx into communication with 

 the exterior. Such animals are Fishes, Amphibia, 

 Reptiles, Birds and Mammals. 



Now besides these animals there are others, which, 

 while not displaying all the above-mentioned charac- 

 teristics, yet betray their affinity to the Vertebrate 

 class by the possession of some of them. 



We designate these animals in common with the 

 Vertebrata as Chordata, and they together with the 

 true Vertebrata or backboned animals constitute 

 the Phylum Chordata. It is among these animals, 

 as representing a lower grade of organization than 

 the Vertebrata, that we might expect to find some 

 indication of the origin of the Chordata from an 

 Invertebrate stock; we will therefore consider them 

 in some detail. 



One class of these lower Chordates, the Ascidians, 

 we may dismiss with a few words, because it will be 

 evident that they owe their simplicity of structure 

 chiefly to degeneration incident on a special mode 

 of life. 



The Ascidians, known popularly as Sea-squirts by 



