ORGANIC STRUCTURE: MORPHOLOGICAL 43 



12. ON THE ORGANS OF THE ANIMAL BODY 



The general motions of the animal body are the expressions 

 of its behaviour, (This we consider more fully in a later Chapter.) 

 These motions of the animal, as a whole, are made possible by 

 the functioning of organs, which we regard, in the meantime, 

 as quasi-independent mechanisms, integrated in various ways. 

 Organs have morphological structure which it is convenient 

 (from the point of vew of exposition) to study just as we have 

 dealt with the animal structure as a whole. We consider, in a 

 very summary way (i) the apparatus of movement ; (2) organs 

 of nutrition ; (3) organs of respiration ; (4) organs of circulation ; 

 (5) glandular organs ; (6) organs of the nervous system and 

 (7) sense-organs. But in a later Chapter we shall deal more fully 

 with the nervous and sense-organs, and organs of reproduction 

 form the subject of a separate Chapter. 



12a. The Appar.\tus of Movement. This involves contractile 

 tissues arranged, with skeletal parts, blood-vessels and nerves, 

 as mechanisms appropriate for the particular motions in question. 



In such a mechanism there is usually a part that moves (" mov- 

 ing bone " in Fig. 10, i), and a part that is relatively fixed (the 

 '* fixed bone " in Fig. 10, i). Flexor and extensor muscles, the 

 contractile tissues, move the part in opposite directions. Blood- 

 vessels carry food materials to the muscle and carry away waste 

 products ; these vessels also carry the oxygen necessary for 

 the energy-transformations. Nerves transmit stimuli from the 

 central nervous system to the muscles and blood-vessels. The 

 skeletal parts to which the muscles are attached may be bones 

 (in vertebrates), shells and carapaces, etc., in the invertebrates. 

 Or there may be no skeletal parts (as in the " bells " of Medusae, 

 the iris of the eye, the walls of arteries, etc. The mechanisms 

 vary in countless ways, but the parts are always such as we 

 indicate here. The mechanisms are parts such as hands, limbs, 

 claws, teeth and jaws, spines, etc. — that is organs for the purposes 

 of locomotion, prehension, eating, aggression, etc. 



Fig. 10, 2, shows the opening and closing mechanisms of a 

 bivalve mollusc, that is, the ligament, or spring, that forces open 

 the shell and the adductor muscle that closes the same. In 3 is 

 shown the muscular and liquid (blood) pressure mechanisms that 

 expand and retract the tentacles, proboscides, etc., of many 



