70 Invertebrata. 



organs or tubes opening laterally, one on each seg- 

 ment. The egg-producing organs are very complex. 



Locomotion takes place by the suckers : the 

 hinder one being fixed, the animal elongates itself 

 and, fixing its front sucker, sets free the hinder one, 

 then shortening its body it proceeds in a similar manner. 

 Leeches can also swim, and when so progressing the 

 body becomes flattened by the contraction of vertical 

 muscular fibres which run from the dorsal to the ven- 

 tral surface, and then by undulating movements it 

 advances like a wavy ribbon. 



Medicinal leeches are principally imported from 

 Hungary and Sardinia. 



CHAPTER XIII. 



NON-PARASITIC WORMS. 



CLASS VIII. Bristle-footed Worms (Chsetopoda). 

 We can scarcely turn over a stone on the sea shore 



FIG. 43. 



Transverse section of a Worm, of Amphioxus, and of a Vertebrate contrasted. 

 a, outer or skin layer ; b, dermal connective layer ; c, muscle plates ; 

 d, segmental organ ; h, arterial, and i, venous blood-vessel ; g, intes- 

 tine ; /, notochord. 



without finding under it some species of the group of 



