282 



LECTURE XXI. 



which require for their retraction special muscles attached to the 

 valves of the shell : in the Pholas crispata there is a third small ac- 

 cessory gill on each side. 



In the Venus chione i^fig. 118.) we have an example of one of 



Venus chione. 



these highest organised of the acephalous Mollusca, in which tlie 

 characters of its grade of structure are indelibly impressed upon the 

 valves of the shell. The muscular fibres (a, a) which retract the 

 margins of the mantle have their fixed point within the margin of the 

 shell, and leave there a linear impression at «' : the adductor muscles 

 leave deeper impressions: b is the anterior adductor, and b' its corre- 

 sponding impression : c is the posterior adductor, and c" its cor- 

 responding impression : d is the retractor of the siphons, and d' its 

 corresponding impression : e shows the transverse fibres of the foot, 

 which, by thin contraction, lengthen the organ, and cause it to pro- 

 trude : / marks the longitudinal fibres or retractors of the foot. 

 When the siphons are present, they are always two in number, corre- 

 sponding with the oral and anal apertures in the tunic of the 

 Ascidian : in Jiff. 118., ff is the inhalent, and ff' the exhalent, siphon. 

 The labial processes are shown at h : the stomach at i. A remarkable 



