L AMELLIBRANCHI AT A. 281 



continued along the interspace of the branchiae towards their extre- 

 mities which are furthest from the mouth. 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 trans- 

 mits 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, 

 distributes branches to the mantle, to the gills, and sends forwards 

 two long filaments, parallel with each other, one on each side of the 

 visceral mass, to the sides of the mouth, where they form small 

 ganglions, communicating with each other by a transverse branch 

 above the mouth, supplying the labial processes, and forming a second 

 feeble communicating arch beneath the mouth, from which the 

 gastric nerves are continued. 



Most of the Lamellibranchiate Bivalves are free and locomotive. 

 The instrument by which they move from place to place is a single 

 symmetrical muscular organ developed from the ventral surface of 

 the visceral mass. The body and protecting shell is longer in pro- 

 portion to its depth in these locomotive bivalves ; and there are two 

 muscles provided for closing the valves. The superadded one is an- 

 terior to the mouth ; the analogue of that which exists in the oyster 

 being the posterior adductor. 



The bivalves with one adductor muscle are termed " monomyaries ; " 

 those with two adductors " dimyaries." The dimyary bivalves have 

 always a foot ; in its least developed condition it is subservient to 

 the function of a gland which secretes a glutinous material analogous 

 to silk, the filaments of which serve to attach certain bivalves, as the 

 Pinna and the common mussel, to rocks ; these filaments are termed 

 the "byssus." 



In most dimyary bivalves the foot is a true organ of locomotion. 

 To some which rise to the surface of the water, it acts, by its expan- 

 sion, as a float ; to others it serves, by its bent form, as an instrument 

 to drag them along the sands ; to a third family it is a burrowing 

 organ ; to many it aids in the execution of short leaps. 



We may generally observe in relation with the greater development 

 and more active functions of the foot, a corresponding complexity of 

 the respiratory system. This is generally eff'ected by the superaddition 

 of accessory organs in the form of tubular prolongations of certain 

 parts of the margin of the mantle, which are provided with a 

 special development of muscular fibres forming tubes called ''siphons," 



