CLASS V. : FISHES (PISCES). 



Aquatic vertebrates of variable body temperature (cold-blooded). Body 

 usually covered with scales. Breathing by gills. Limbs, when present, 

 having the form of fins ; possessing also unpaired fins placed in the 

 middle line of the body. Mostly oviparous. 



1. Form of Body. — -All fish inhabit the water. Their life is passed in 

 a medium which cannot be divided with the same ease as the more fluid 

 air. Accordingly, in all fishes, which — as is the case with the vast 

 majority — on account of their predatory mode of life must be endowed 

 with the capacity of rapid locomotion, the body is laterally compressed and 

 tapering — often even pointed — at its anterior and posterior extremities ; 

 it bears, in fact, a strong resemblance to a spindle or the hull of a boat, 

 and is therefore excellently adapted for cutting through the water. 

 (Compare, on the other hand, fishes living on the bottom— eels, plaice, 

 rays.) How is this spindle shape produced ? 



(a) The head and body are immovably united; a movable neck 

 would, in fact, be an impediment to forward movement. (Why? See 

 Section 2.) Nor is a neck required, fishes being able to move their 

 head and body together with ease in any direction they desire. 



(b) In most vertebrates the tail is more or less distinctly marked off 

 from the body (examples) ; in fishes, on the other hand, the body along 

 its whole breadth passes gradually into the tail, which commences with- 

 out any break at the posterior termination of the abdominal cavity (see 

 Section 2, b). 



(c) The "pectoral and pelvic girdles, by which the breadth of the body 

 in most other vertebrates is much increased at its anterior and posterior 

 extremities, are in fishes very feebly developed. (Why ? See Section 4.) 



2. Motion and Locomotive Organs. — (a) The rudder and oars of a 

 boat are provided with broad surfaces, for the purpose of enabling man 

 to exert a strong pressure against the water. The limbs of fishes are 

 constructed on a similar plan, presenting flat surfaces, while the portion 

 connecting them with the trunk (i.e., the handle of the oar) is usually 



