The Classification of Animals. 201 



Some of the main characters of these four classes will now 

 be given. 



Cla^s I. HEMICHORDATA. 



Sub-class (a). Enteropneusta. — This sub-class contains only 

 the genus Balanoglossus, a worm-like organism. There is a 

 large proboscis, and the gill-slits extend a long way down the 

 body. The notochord, which is an at first hollow, but after- 

 wards solid, outgrowth of the intestine, only exists in the 

 region of the collar and proboscis. The dorsal nerve-cord is 

 imbedded in the skin, and a ventral cord also imbedded in 

 the skin. The coelom is spacious, but there is nothing that can 

 be definitely compared to nephridia. There productive organs 

 are sacs imbedded in the body-wall, and opening on to the 

 exterior. 



Sub-class (b). Cephalodiscida. — To this sub-class is now 

 assigned an organism {Cephalodiscus), showing many resem- 

 blances to the Polyzoa, with which it was at first confounded. 



Sub-class (c). Rhabdopleurida. — ■ Rhabdopleu7-a is an 

 organism which has also been referred to the Polyzoa. But 

 since, like Cephalodiscus, it has gill-slits and a small noto- 

 chord, it conforms to two out of three essential character- 

 istics of the Chordata. 



Class 2. UROCHORDATA. 



These animals, more generally known as Ascidians or 

 Tunicata, are abundant in genera and species. It is only in 

 the larval stages, and in the persistently larval Appendiadaria 

 and its immediate allies, that the nervous system is in the form 

 of a dorsal tract, and that the notochord is present. This latter 

 is developed only in the tail, thus contrasting with the Hemi- 

 chordata, where it is only found in the head region. 



Class 2i. CEPHALOCHORDATA. 

 To this group belongs only the Amphioxus, of which there 

 are several species. It is an elongated fish-like animal. It 



