398 



ANIMAL RESPIRATION 



BIVALVE MOLLUSCS (Lamellibranchia) 



Bivalves possess vi^ell- developed breathing organs, as may 

 readily be seen by even a superficial examination of any common 

 form. If we take, for example, a Mussel, Cockle, or Oyster, we 



shall find the body covered by a pair of 

 strong calcareous plates, which are the 

 halves of the bivalve shell, and are 

 placed on the right and left sides of the 

 body. These valves cover, and indeed 

 are chiefly secreted by, two flaps of the 

 body -wall, constitutino- together the 



constitutmg 

 mantle-skirt. If a human being were 



clothed in a coat so large that its sides 

 completely covered his body, head, and 

 limbs, he would serve as a rough model 

 of the arrangement — that is, if one could 

 imagine the coat to be a part of him 

 and not merely an investment. These 

 two large mantle-flaps of a bivalve do 

 a considerable amount of the work of 

 breathino- and if one of them be lifted 

 up a large plate-like ^z7/ will be seen 

 beneath it in any one of the three com- 

 mon types named (fig. 530). These 

 gills, the shape of which has suggested the name of the group 

 (Lat. hmel/a, a thin plate; Gk. branchia, gills), share the work of 

 the mantle -flaps, and are usually regarded as 

 equivalent to the two plume -like gills of a 

 cuttle-fish or ormer. We have seen (see 

 p. 248) that bivalves are in some respects 

 degenerate, and depend for food upon ciliary 

 currents, which carry minute organisms to the 

 mouth. Both mantle -flaps and gills are richly 

 covered with cilia, and act as current-pro- 



Fig. 530. — Freshwater Mussel [A7iodoiUa) 

 opened and seen from below 



rt, Position of mouth; bb, adductor muscles 

 [which keep shell closed) cut through ; c, 

 mantle lobe; (/, labial palps or feelers: e^ foot; 

 f, left gill. 



breathing 



Fig. 531. — Freshwater Mus- 

 sel [Anodojita] embedded 

 obliquely in mud, with hinder 

 end projecting. The arrows 

 indicate the currents of water 

 which enter and leave the 

 mantle-cavity 



ducing organs, thus enabling 



feeding and 



breathing to go on, and also providing for the 

 removal of products of waste when the animal is buried in sand 

 or mud, with only the hinder end projecting (fig. 531). We 



