330 



MOLLUSCA. 



ES 



the shell. These are typically two in number (Dimyaria), but in 

 some forms the anterior adductor is smaller than the posterior 

 (Mytilus), and may be absent altogether (Monomyaria), as in Ostrea, 

 Pecten, etc. (Fig. 252). The adductors sometimes consist of two 

 parts of a different aspect : the fibres of one of these parts are said 

 to be transversely striated, and capable of rapid contraction, those 

 of the other being smooth, but this has been denied. (3) The 

 anterior and posterior retractors, which are specialized portions of 



the adductors, serve 

 for the retraction of 

 the animal into the 

 shell; and the pro- 

 tractor which passes 

 from its attachment 

 just behind the ven- 

 tral portion of the 

 anterior adductor. 



The foot, which 

 often contains some 

 of the viscera (intes- 

 tine, liver, and gon- 

 ad), is completely 

 absent in compara- 

 tively few forms, and 

 only in those which 

 have lost the power 

 of locomotion (Os- 

 trea, Anomia, Teredo, 

 etc.). In many 

 forms, principally in 

 the larva, less fre- 

 quently in the adult 

 (Mytilus, Fig. 256), the foot possesses a byssus gland, which secretes 

 silk -like fibres, by which a temporary or permanent attachment 

 of the animal is effected. The form and size of the foot vary 

 very considerably, in accordance with particular kinds of locomotion. 

 The foot is most frequently used for creeping in sand, and is then 

 hatchet - shaped ; in other cases it is spread out laterally, and its 

 creeping surface has the form of a disc. More rarely it is of a 

 large size and bent, in which case it is used for springing movements 

 in the water (Cardium, Fig. 255). Some Lamellibranchs possess a 



M 



FIG. 255. Anatomy of Cardium tuberculatum (after C. Grob- 

 ben). A exhalent siphon ; A auricle of heart ; Af anus ; 

 D intestine ; E inhalent siphon ; F foot ; G generative 

 organ ; Goe generative opening ; HS posterior adductor ; 

 K gill of right side ; L liver ; M right lobe of mantle ; Mg 

 stomach ; N kidney ; mouth ; S right valve of shell ; 

 V ventricle ; VS anterior adductor muscle. 



