2 TEXT-BOOK OF ZOOLOGY 



especially if tapering to a point at its anterior end. Accordingly, the 

 body of a vertebrate animal assumes, to a greater or less degree, the 

 form of a pointed cylinder, and the movements of the animal are 

 accordingly performed for the most part in the direction of its longi- 

 tudinal axis. 



The body of every vertebrate is divisible by one section into two 

 similar halves, a left and a right portion. (Explain how such a section 

 is made through the body of a man, bird or fish.) These halves bear a 

 relation to each other similar to that of an object to its image reflected 

 in a mirror. Hence, vertebrates are laterally symmetrical animals. Of 

 those organs which are present in pairs {e.g., eyes, limbs, etc.), one lies 

 in each half of the body equidistant from the plane of section; the 

 unpaired organs, on the other hand {e.g., mouth, tail), lie in the plane 

 of section. The internal organs also {e.g., lungs, kidneys, oesophagus, 

 trachea) for the most part assume similar positions. 



On the other hand, those internal organs which occur singly — such 

 as the liver, heart, intestine — and which do not lie in the plane of 

 section, are always so placed as to throw an equal weight on both halves 

 of the body. By this arrangement of organs, both body-halves maintain 

 their balance, and an animal constructed on this type is accordingly 

 capable of moving with greatest ease. (What do you observe, in carrying 

 a heavy load, such as a pail of water, on one side only ?) People who 

 have lost an arm frequently wear a wooden arm for the purpose of 

 restoring their lost balance. 



The body of every vertebrate consists of head, trunk and limbs. 

 (State what organs are found on or in the head and in the trunk— chest 

 and abdomen.) As a rule, the head and trunk are united by a neck, 

 which represents, as it were, a movable stalk for the head. The number 

 of limbs is at most two pairs; in some cases both pairs are absent 

 (snakes), in others only one pair occurs (whales). The principal forms 

 of limbs are fins, wings, legs. (Give examples.) What is the function 

 of limbs ? 



2. Skeleton.— The body of every creature must possess a certain 

 amount of solidity, though this, according to our notions, may often be 

 extremely slight {e.g., in infusoria, the threads or mycelium of fungi). 

 Soft-bodied organisms like this, however, can only live in water, in the 

 earth, or within other bodies, where their own bodies may be supported 

 and protected on all sides. Deprived of this support, the creature 

 shrivels up and perishes {e.g., a jelly-fish, thrown up on the shore). 

 Hence, animals living on land or in the air, as most vertebrates do, 

 require a greater amount of solidity. This is attained by the possession 

 of an internal bony (or cartilaginous) framework, the skeleton (see p. 9), 



