2 3,0 



SYSTEMATIC ZOOLOGY 



in the integument. On the ventral side of the body is a longi- 

 tudinal groove, in which a narrow ciliated ridge is generally 



present and cor- 

 responds to the 

 foot of other 

 Mollusca. At 

 the posterior 

 end of the body 

 there is gener- 

 ally a pair of 

 gills near the 

 anus. With the 

 exception of one 



genus they are hermaphroditic. The order presents both primi- 

 tive and degenerate characteristics. 



FlG. 230. Proneomenia sluiteri, one of the Solenogaslres. Lateral 

 : above, ventral aspect below ; natural size about ten centi- 

 meters long. (From Lang's Comparative Anatomy.) 



Order 2. Placophora 



The Placophora (Gr. 7r\af, plate, and (f)opea>, to bear) are 

 represented by the chitons (Figs. 231 and 232). The most strik- 

 ing external feature of these 



animals is the shell on the 

 dorsal side of the body, 

 which consists of eight over- 



.*/••, 



Fie. 231. Chiton spinosus, dorsal 

 aspect. (From the Cambridge 

 Natural History. I 





Fir,. 232. Chiton, showing the eight overlap- 

 ping plates of the shell. (After Pretre, from 

 Lang's Comparative Anatomy.) 



