Phylum Chordata 575 



Other Cyclostonies. — In addition to the parasitic lampreys, there 

 is another group of fresh-water forms, the brook lampreys. Their 

 life history is similar to that of the sea lampreys, but the adults do not 

 feed. A fleshy hood extends over the mouth and the adults simply 

 breed and die. These abundant animals of our small streams should 

 not be confused with the parasitic lampreys as they do no harm to any 

 fish. 



The hagfishes are marine forms, often termed the myxinoids due 

 to their ability to produce enormous amounts of slime. In general, the 

 hagfishes should be considered as scavengers, inasmuch as they feed on 

 dead or dying fish. Often they cause considerable loss to fishermen by 

 attacking fish caught in nets. In feeding, they enter through the anus 

 of the fish and clean out the entire visceral mass, leaving only the empty 

 outer skin and bones. 



The hagfishes have an unusual gonadal development. There is a 

 common sex organ in each individual, but only the male or the female 

 portion develops. One portion thus is functional, while the other 

 remains rudimentary. 



THE CLASS CHONDRICHTHYES 



Those fish possessing true jaws are divided by ichthyologists into 

 two main classes : the Chondnchthyes including the sharks, rays, and 

 chimaeras, and the Ostcichthycs containing the true bony fish. The 

 elasmohranchs, as the members of Chondrichthyes are known, are a very 

 ancient group as judged by their fossil record. They date from the 

 Silurian era and have been abundant since that time in the ocean. Only 

 a few have managed to survive in fresh-water lakes ; in such cases, the 

 sharks have not invaded the fresh water, rather they have been iso- 

 lated by geologic changes. These land-locked sharks are found in Lake 

 Nicaragua in Central America and in some of the lakes of the African 

 Rift Valley. 



The Characteristics of the Class. — All the members of this class 

 have skeletons composed of cartilage and a persistent notochord. The 

 vertebrae, however, are separate and complete. The mouth is ventral 

 in position and is provided with numerous teeth capped with a hard 

 dentine. The tough skin has placoid scales (Fig. 22,C) and many glands 

 which secrete mucus. There are paired pectoral and pelvic fins as well 

 as unpaired dorsal and caudal fins. The pelvic fins of the male possess 

 claspers. There are but two chambers in the heart : a single auricle and 



