HIGH SCHOOL ZOOLOGY. 



11 



Diffoi'ont regions of tlio boily have different duties to dis- 

 clmrge, and consequently diiler in form and structure. We 

 distinguish in a fish the head, trunk, and tail, of which the first 

 lodges the brain and sense organs, secures food, and shelters the 

 gills, the last is chiefly locomotive in function, while the trunk 

 differs from Vjoth in being hollowed out so as to enclose the in- 

 testines and other viscera in the so-called body-cavity (ccelom). 

 The regions referred to are said to be axial, because they are 

 disposed round the chief axis of the body, while the two pairs 

 of limbs or appendages, much more developed in the higher 

 Vertebrates, project laterally from the trunk, to which they are 

 attached in the neighbourhood of the cephalic and caudal 

 regions respectively, and ai-e described as appendicular. 

 In a fish the anterior and posterior appendages are known as 

 the pectoral and ventral fins, (Fig. 1) from which are to bo 



Fig. 1,— Conimou Catfi=>h, or Bullhead J 

 Amiurtis tieMdosus. 



distinguished the unpaired fins, occupying the middle line 

 of the dorsal and ventral aspects of the trunk and tail, and as- 

 sisting in locomotion. Tlie latter are named from their posi- 

 tion dorsal, caudal and anal. In the catfish, part of the dor- 

 sal is separated as the adipose fin, which is regarded as the 

 rudiment of a longer dorsal, and, instead of being supported 

 by fin-rays, has only fatty tissue within it. 



4. A'OertureS, — Certain apertures exist on the. surface of 

 the body ; of these, the mouth is bounded by tlio upper and 

 lower jaws and leads into the mouth-cavity, the nostrils or 



