LESSONS IN ZOOLOGY. 291 



The individuals of this group are small, and are found usually in 

 the sands of shallow water along the seashore. 



The Marsipobranchia, are vermiform animals with a cartilagin- 

 ous skeleton, and persistent notochord. The mouth is suctorial ; 

 the alimentary canal a simple tube. The blood is red, and the 

 heart has two valves. The brain is distinct, with three sense 

 nerves: the optic, auditory and olfactory. They have dorsal 

 and caudal fins, but no scales. Respiration by means of five or 

 six pairs of gill sacs. They are all parasites, including the marine 

 hagfish, and the lampreys, living in both fresh and salt water. 

 They attach themselves to dead or living fish, and also eat worms 

 and other small aquatic animals. 



The Selachii have a cartilaginous skeleton, pectoral and ven- 

 tral fins, transverse mouth, and a capacious stomach. Respira- 

 tion is accomplished by means of one or several gill openings. 

 There is no air bladder, and the arterial bulb has three series of 

 valves. The nervous system is well developed, but the optic 

 nerves do not decussate. This group includes the different kinds 

 of sharks, dog fishes, skates and rays. They are carnivorous, 

 and generally marine. 



The Ganoidei and Teleostei are regarded as true fishes. The 

 Ganoidei have a bony or cartilaginous skeleton with enameled 

 scales or osseous plates. The arterial bulb is muscular with 

 numerous valves, and there is a partial decussation of the fibers 

 of the optic nerve. The sturgeons, gar-pikes and mud-fish are 

 the chief representatives of this group in the waters of the present 

 time, but earlier in the history of the earth they were the domi- 

 nant form of fish life. 



The Teleostei are bony fish, and include the great bulk of the 

 fishes of the present time ; they are the typical fish, and perhaps 

 we can best learn something of them by a somewhat detailed 

 study of some common fish, as the yellow or ringed perch ; or a 

 bass or a croppie would serve as well. 



Holding a fish in its natural position, notice that it is symmetri- 

 cal , has an anterior and posterior region, a dorsal and ventral sur- 

 face and a right and left side. A fish may be cylindrical, or flat- 



