504 



LECTURE XXI. 



a single symmetrical muscular organ {ih. b) developed from the 

 ventral surface of the visceral mass. The body and protecting 

 shell {ib. a) is longer in proportion to its depth 

 in these locomotive bivalves ; and there are tvv^o 

 muscles provided for closing the valves.* The 

 superadded one {ib. c) is anterior to the mouth ; 

 the homologue of that which exists in the oyster 

 being the posterior adductor (ib.f). In all the 

 present class the divarication of the valves is 

 provided for by the insertion of an elastic sub- 

 stance at their hinge | ; and the valves are drawn ' 

 together and closed by the contraction of the ad- 

 ductors. The bivalves with one adductor muscle 

 are termed " monomyaries ;" those with two ad- 

 ductors "dimyaries." The dimyary bivalves have 

 always a iooi {Jig. 188, b) : in its least developed 

 condition it is subservient to the function of 

 a gland which secretes a glutinous material ana- 

 logous to silk, the filaments of which serve to at- lenma. 

 tach certain bivalve?, as the Pinna and the common mussel, to rocks ; 

 these filaments are termed the " byssus {Jig. 191, //, o)." The visceral 

 mass occupies about half the cavity of the shell next the hinge. The 

 rest of the interspace of the pallial lobes is almost wholly occupied by 

 the branchial laminas (Jig.l88, e, e), which are usually four in number, 

 of a crescentic figure, placed two on each side of the visceral mass. 

 This is the characteristic condition of the respiratory organs in the pre- 

 sent class of Acephalous Mollusca, and from which it derives its name. 



In the oyster the mouth opena beneath a hood-shaped fold of the 

 mantle, at the middle of the base of the labial processes : the canal 

 is continued by a short oesophagus to an expanded stomach, from 

 which numerous ramified hepatic follicles are developed. The in- 

 testine, after describing a few convolutions, is continued along the 

 branchial interspace towards the extremities of the branchise which 

 are furthest from the mouth, where it opens into the pallial cavity. 

 The ovarium or the testis surrounds the intestinal convolutions, and 

 forms with the liver the chief part of the visceral mass. 



The veins of the oyster terminate in a single auricle, which 

 transmits the blood to a pyriform ventricle ; the two divisions of the 

 heart being contained in a distinct pericardium, situated between the 

 visceral mass and the concave margin of the adductor muscle. 



The principal centre of the nervous system lies upon the opposite 

 convex margin of the same muscle, supplies it with nervous influence, 



• Preps. Nos. ■'51 and .52. 



t Preps. G6, 67, X. vol. i. p. 16. 



