Introduction to Animal Morphology. 261 



a tongue-like process, grooved at its base, single or lobed, 

 having part of the foot retractor, or a special muscle inserted 

 into it as a retractor ; the form of the threads depends on the 

 grooves of the byssus organ, and we may find them each with 

 a rind, and consisting of separate threads ( Mytilus, Malleus, 

 Dreissensia, Pecten), or with a rind, but not consisting of 

 threads (Area), or with no rind and its root broadly expanded 

 (Lima, Meleagrina), or without an expanded root (Pinna, 

 Perna). A byssus gland may exist in the embryo, vanishing 

 in the adult (Unio,Cyclas). The shelly plug whereby Ano- 

 mia is anchored seems to be of this nature, and a retractor 

 muscle is attached to the plug. The foot and other parts 

 secrete phosphorescent mucus in Pholas and Lithodomus. 

 The foot is the organ of locomotion. Lyriodon has been 

 seen jumping four feet high by its contraction. 



The shell is closed by adductor muscles which 

 pass transversely from one valve to the other ; they 

 are attached to scar-like muscular impressions con- 

 spicuous on each valve, and are usually two in num- 

 ber [Dimya] ; sometimes close together, or even fused 

 (Tridacna). In Miilleria and ^theria there are two 

 adductors in the embryo and only one in the adult ; 

 others [Monomyd) have but one (posterior) adductor. 

 In Anomia there is a third muscular scar, hence the 

 name Trimya has been given to it. The two in Dimya 

 may be similar in size [Isoinya], or form [Homomya] or 

 ^\ss\mi\ax [Heteromya). Some, as Pecten, with small 

 foot, swim by alternate closure and opening of the 

 shell. 



The heart lies in a part of the body cavity called 

 the pericardium,"^' under the hinge line and above and 

 behind the liver ; it is lined by tesselated epithelium. 



* Anomia has no pericardium. 



