226 CLASSIFICATION OF VERTEBRATES. 



eels, snakes, etc., they have been lost. The skeleton is well 

 developed, the vertebrae being cartilaginous or osseous, while the 

 skull is more or less completely roofed in with cartilage or bone. 

 The gill slits are narrow clefts, and are never more than seven 

 in number, and the branchial arches are well developed. The 

 gnathostomes are subdivided into two great divisions or grades, 

 the Ichthyopsida and the Amniota. 



GRADE I. ICHTHYOPSIDA (ANAMNIA, 

 ANALLANTOIDEA). 



As the name implies, the ichthyopsida include the fish-like 

 forms characterized by the presence of functional gills, either 

 in the larval stages or throughout life, the absence of an 

 amnion and true allantois (see amniota), and the presence in 

 young or adult of median fins. 



The surface of the body is rich in glands, and in most forms 

 the skin contains scales largely of dermal origin (p. 92). -In 



^ the young the skin also bears 



sense organs belonging to the 

 lateral line system (p. 67) in- 

 nervated by the seventh and 

 tenth nerves ; but in those' 

 forms in which the adults as- 

 FIG. 232. Diagram of skin of a fish, sume a terrestrial life (am- 



c, cuticle j^dermis;,, epidermis, con- phibia) this s stem becO mes 

 taming, g, epidermal glands. 



lost upon leaving the water. 



In the young, median fins, formed by a fold of the integument, 

 occur ; and in the fishes these are supported by rays of dermal 

 origin, or by true skeletal rays, or by both. In the amphibia 

 the rays are lacking, and the fins themselves disappear on the 

 assumption of a terrestrial life. 



In the skeleton the noticeable features are the small size, or 

 absence, of the basisphenoid, and the large size of the parasphen- 

 oid when present. Usually (fishes) no sternum is present ; in 

 the amphibia, where it occurs, it is never connected with the 

 ribs. The branchial arches are four or more in number, and 

 these are largely persistent in the adult. 



