WORMS CONSIDERED AS A CLASS. 371 



nc 



Transverse section oi a Worm. 



*, skin or body wall ; nc, double nervous cord or nervous ganglions ; ac, aliment- 

 ary canal ; ft, heart or dorsal vessel ; to, locomotive organs or tubercles tipped 

 with bristles or hairs; nc', supplementary nervous cord ; ft', supplementary circulatory 

 vessel. 



bristles (Fig. 505), or plates which serve as locomotive 

 organs. Those provided with bristles or setse, are called 

 Chaetopods, from the Gr. chaite, hair, and pous y a foot. 



Their digestive system is very simple. Their circula- 

 tory system is but little more than a dorsal vessel or so- 

 called heart. Their blood in many cases is red. They 

 respire by means of branchiae, or by the whole surface of 

 the body. Their nervous system, even when present, is 

 nearly equally distributed among the different segments. 



The Annelida have the sense of touch highly developed, 

 and many are provided with tactile organs. As to sight, 

 in many (Serpula, jLumbricus, etc.) eyes are wholly want- 

 ing; in others there are two or more very distinct eyes. 

 Organs of hearing have been detected in some ; and it is 

 believed that many can perceive sounds. 



Worms multiply by means of eggs ; and some kinds also 

 increase by gemmation or fission that is by self-division 

 (Fig. 506). A few are viviparous. In most kinds the sexes 

 are distinct; but in some kinds (Earthworms, Leeches, 

 etc.) the two sexes are united in the same individual. 



Worms live in the water, in mud, in damp earth, and 



