140 THE BIOLOGY OF AN ANIMAL. 



in the ganglia (C*), Avhich thereupon send forth efferent impulses to the 

 appropriate muscles. Orderly contractions result, and the worm draws 

 back from the unwelcome irritation. Within the nerve-centre some dis- 

 criminating action takes place, such that the afferent impulses are not 

 merely " reflected," as the word "reflex" might seem to indicate, but are 

 worked over and are sent as efferent impulses to only those muscles neces- 

 sary to effect the appropriate action. Little is known of the nature of this 

 discriminating function, but it is certainly unconscious in a vast majority 

 of cases. For a fuller discussion the student is referred to special works 

 on Physiology. 



Sensitive System. We have learned (p. 118) that the earthworm 

 possesses the senses of touch, taste, sight, and smell, although it 

 has no special organs for these senses, and hence can hardly be said 

 to possess any proper sensory system. We do not know, more- 

 over, whether the so-called "senses" of the earthworm are really 

 states of consciousness as in ourselves, for we do not even know 

 whether earthworms possess any form of consciousness (compare 

 p. 6). When, therefore, we speak of the earthworm as possess- 

 ing the " sense" of touch or of sight we mean simply that some 

 of the nerves terminating in the skin may be stimulated by me- 

 chanical means or by rays of light, without necessarily implying 

 that the worm actually feels or sees as we feel and see. 



The sense of touch extends over the whole surface of the 

 body. That of taste is probably located in the cavity of the 

 mouth and pharnyx ; the location of the sense of smell is unknown. 

 Darwin's experiments have shown that the earthworm's feeble 

 sense of sight is confined to the anterior end of the body. It is 

 probable that the nerves of sight, taste, and smell enter the cere- 

 bral ganglia alone, while those of touch run to other ganglia as well. 



Systems of Support, Connection, Protection, etc. The structure 

 and mode of life of many animals are such as to require some 

 solid support to the soft parts of the body. Such supporting 

 structures are, for instance, the bones of vertebrata, the hard 

 outer shell of the lobster, or beetle, and the coral which forms 

 the skeleton of a polyp. The earthworm has, however, nothing 

 of the sort, and it is obvious that a hard supporting organ would 

 be not only useless, but even detrimental. The power of creeping 

 and burrowing through the earth depends upon great flexibility 

 and extensibility of the body ; and with this the presence of a 

 skeleton would be incompatible. 



