GENERAL GHARAOTERS OF FISHES. 411 



Lahm'ntory Work. — The anatomy of these animals is exceedingly in- 

 teresting ; the respiratory sacs and nasal duct can be exposed by a lon- 

 gitudinal section of the head ; the relations of the notochord can be 

 best seen by transverse sections ; the heart and vessels should be in- 

 jected. Preparations of the brain should be made, and with care the 

 skull prepared. 



Class III. Pisces {Sharks, Rays, Sturgeons, Oarpikes, and 

 hony fishes). 



General Characters of Fishes. — We now come to Verte- 

 brates which have genuine jaw-bones and paired fins, and 

 which, in short, are affiliated to the Batrachians, and through 

 them with the reptiles, birds, and mammals. All the fishes 

 agree in having a true skull, to which is attached a movable 

 lower jaw. The brain is well developed, with its lobes for 

 the most part, at least, equivalent to or homologous with 

 those of the reptiles, birds, and mammals, though the cere- 

 bral hemispheres are small, and in most fishes of nearly the 

 same size as the optic lobes ; the cerebellum is also generally 



Boi'^al jin. 



Caudal. 



Anal. Ventral. Pectoral. 

 Pig. 388. - The Mud-Minnow. 



of moderate size. The head forms part of the trunk, there 

 being no neck (except in the Hipiwcampidce), and the body 

 is usually compressed and adapted in shape for rapid motion 

 in the water. 



Paired fins are always primitively developed, though the 

 posterior or ventral fins, at least, are in many cases wanting 

 through the atrophy of parts developed in embryonic life. 

 The pectoral and ventral fins (Fig. 388), which represent the 

 fore and hind legs of higher Vertebrates, are attached to the 

 body or trunk by a shoulder and pelvic girdle. The shoulder 



