Chap. III.] 



MOLLUSC A. 



Si 



or asymmetrically developed. The appearance of this 

 torsion allows us to divide the Gastropoda into a 

 lower or more primitive, and a higher or more 

 differentiated series.. 



a. Isopleura. In these the two sides of the 

 body are equally developed, and 

 many of the characters of the 

 primitive mollusc are retained 

 unchanged. Here we have the 

 Polyplacopliora, represented 

 by the Chitons, in which the shell 

 is broken up into eight pieces ar- 

 ranged in a fashion to which it is 

 difficult to refuse the name of 

 metameric arrangement (Fig. 38) ; 

 and the TVeomeniidse, and the 

 Clitrtodermatidti 1 , in which 

 the shell is represented by spicules 

 only. 



. In the Aiiisopleura we 



have an exceedingly interesting phenomenon ; while 

 the body undergoes torsion, the nerve-cords that run 

 down the sides of the body may or may not be impli- 

 cated in the change. Where they are not we have 

 the Etithyiieura, which either, like Aplysia and 

 Doris, continue to breathe by gills the oxygen dis- 

 solved in water, or like the pond-snail (Lymnoeus), 

 the garden-snail (Helix), and the slug (Lirnax), have 

 their gills aborted, and a breathing chamber or lung 

 formed by the apposition of part of the edge of the 

 mantle to the side of the body. 



In the Streptoneura the nerve-cords are impli- 

 cated in the general torsion of the body, and form a 



V ' 



figure of eight loop ; in the Zygobranchiata, of 

 which the ear-shell (Haliotis) and the limpet (Patella) 

 are examples, the right and left gills become re- 

 spectively the left and right, and are equal and 

 c 16 



Fig. 38. Chiton mag- 

 nificus. 



