288 CLASSIFICATION OF ANIMALS 



tubular infolding of the ectoderm of the dorsal side. The 

 axial skeleton consists primarily of a notochord which persists 

 in the lower forms but is only found in the larval stages of the 

 higher forms. In addition a vertebral column and a skull 

 supplement or replace the notochord. The vertebral column 

 consists of a series of segmentally arranged vertebrae with dorsal 

 arches protecting the nervous system and ventral arches pro- 

 tecting the viscera. 



627. Class I. Cyclostomata. The round mouth eels. This 

 class comprises only a few species of eel-like animals which 

 are destitute of a lower jaw. The skin is smooth, i. e., there 

 are no scales. There are no paired appendages. There is a 

 median dorsal fin which is continued around the tip of the tail 

 forming a tail fin. The mouth forms a circular sucking disc 

 which is covered with hard epidermal tubercles by which the 

 animal bores through the skin of the host to which it attaches 

 itself. The Cyclostomes are ectoparasites and some even make 

 their way for some distance into the host. The alimentary 

 canal is practically a simple tube, though some forms have a 

 spiral valve; and there is a large liver which opens into the 

 digestive tract by a duct. There are also glands in the wall 

 of the intestine. There are 6-14 pairs of gill slits which open 

 directly to the exterior. The skeletal system consists of a 

 well-developed notochord with a thick fibrous sheath, and a 

 number of cartilages. The brain is enclosed in a skull composed 

 partly of cartilage, partly of membrane. To this are attached 

 the two cartilaginous ear capsules and a cartilaginous nasal 

 capsule. There is also a network of cartilages surrounding and 

 supporting the mouth and pharyngeal regions. The vertebrae 

 consist only of neural arches with intercalary pieces, and of 

 haemal arches in the tail region. There is a heart similar to 

 that of fishes. The olfactory organ is a single sack with a 

 median opening. The eyes are of the typical vertebrate type 

 but in some cases more or less reduced. The ear is a simple 



