70 THE BIOLOGY OF AN ANIMAL. 



hence can hardly be said to possess any proper sensory system. 

 We do not know, moreover, whether the so-called "sensations" 

 of the earthworm are really states of consciousness as in ourselves, 

 for we do not even know whether earthworms possess any form 

 of consciousness. When, therefore, we speak of the earthworm 

 as possessing the ' ' sense ' ' of touch or of sight we mean simply 

 that some of the nerves terminating in the skin may be stimu- 

 lated l)y mechanical means or by rays of light, without necessa- 

 rily implying that the worm actually feels or sees as we feel and 

 see. 



It has recently been shown that the skin contains many cells each of 

 wliich gives off a single nerve-fibre that may be traced directly into the 

 ventral nerve-cord. These "sensory cells " may be regarded as "end- 

 organs" through which the stimuli are conveyed to the fibres. It has also 

 been shown that tliese cells are aggregated in minute groups thickly scat- 

 tered over the surface of the body. Each of these groups may be regarded 

 as a simple form of sense-organ. 



The sense of touch extends over the whole surface of the 

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

 month and pharnyx; the location of the sense of S7nell is un- 

 known. Darwin's experiments have shown that the earth- 

 worm's feeble sense of sight is confined to the anterior end of 

 the ])ody. It is probable that the nerves of sight, taste, and 

 smell enter the cerebral ganglia alone, while those of touch run 

 to other ganglia as well. 



Systems of (Organs 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 sup- 

 porting-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 might be incomjmtible. 



The connecting system consists simply of various tissues by 

 which the different organs are bound firmly together. These 

 can only be seen upon microscopical examination. The most 

 important of them is known as connective tissue. 



