100 VERTEBRATE ZOOLOGY 



pressed types, the broad, shallow, bottom-feeding types, and many 

 minor types. 



Structural Features of the Fishes 



The fish in its typical form is essentially an aquatic mechanism — a 

 submarine automaton. Like a submarine vessel it has a fusiform 

 shape; steering and balancing devices; hydrostatic appliances, such 

 as air reservoirs; a mechanism for extracting oxygen from the water; 

 optical devices adapted for aquatic vision; and instruments for de- 

 tecting vibrations in water, warning of the approach of the enemy or 

 of the nearness of prey. 



The following formal characterization of the Class Pisces will serve 

 to distinguish them from other classes: 



1. Jaws: fishes proper are all Gnathostomata (hinged-mouthed 



as distinguished from the Cyclostomata or round-mouthed 

 eels. 



2. Gills or Branchiae. — Their method of respiration is distinctly 



aquatic throughout life, though accessory organs for air- 

 breathing occur in several distinct orders of fishes. The gills 

 are vascular processes of the walls of the branchial clefts. 



3. The Circulation is built about the gill system. The blood is 



pumped forward from the ventral heart through the gills and 

 is thence, as arterial blood, carried backward in the dorsal 

 aorta. This scheme of circulation wherever found will be 

 interpreted as primarily aquatic. 



4. The Heart is a single S-shaped muscular tube with but one 



auricle, one ventricle, and a bulbios arteriosus, and receives 

 only venous blood. 



5. Tail.— The principal organ of locomotion is the tail, terminated 



by a paddle-like expansion, the caudal fin, and sculled by 

 means of the powerful segmental muscles. 



6. Fins.— These are of two sorts, the paired and the median fins. 



The paired fins, pectoral in front and pelvic behind, are homol- 

 ogous to the fore and hmd limbs of terrestrial vertebrates 

 and are supported by bony or cartilaginous rays articulated 

 with a simple pectoral or pelvic girdle, which may be either 

 bony or cartilaginous. These appendages are essentially 

 balancing organs though they may be modified for various 

 purposes, or even lost. The median fins occur in both ven- 



