FISHES 225 



brain of fishes is well developed and the blood is red but 

 cold. They have a true but single heart consisting of an 

 auricle and ventricle. Most fishes breathe only by means 

 of gills. The gills vary in number and structure in different 

 orders, but are alike in their general structures in the bony 

 fishes, of which those described in the perch may be taken 

 as an example. 



Habits and adaptation to environment. — Fishes are 

 preeminently adapted to an acjuatic life, and all live in the 

 water. The body, in most cases, is of such a form that it 

 "cuts" the water and thus offers the least resistance. 



For obtaining air from the water, all fishes are furnished 

 with some kind of gills. This is a direct adaptation to their 

 environment. Again, the fishes are cjuite large animals and, 

 for the most part, are very active. Consequently, there is 

 a great deal of the tearing down and buikling up processes 

 going on in the body. As a result, there nrust be some adap- 

 tation or arrangement whereby the blood may have free, 

 constant, and full access to air. To provide for this we have 

 already seen how the mouth, pharynx, and gill covers are 

 so constructed that a current of fresh water, laden with air, 

 is kept constantly flowing over the gills. Moreover, the 

 gills are not of one piece, but are composed of a great number 

 of filaments to proviile more space whereby the blood may 

 come in contact with more water. Again, the limbs of 

 fishes, if the fins may be called such, are admirably adapted 

 for locomotion in the water, but totally useless on land. 



Some fishes are protected from their enemies by their 

 resemblance to objects in the sea. One of the anglers is 

 said to counterfeit very remarkably a rock with its attached 

 growth of seaweed and sea animals. Undoubtedly this 

 resemblance also serves as a ruse for capturing its prey. 



heukick's ZOOI,. Vt 



