320 



COMPARATIVE ZOOLOGY 



tufts along the side of the body ; as these float in the 

 water, the blood is purified. 118 Bivalve mollusks have 

 four flat gills, consisting of delicate membranes filled 

 with blood vessels and covered with cilia. In the 

 oyster, these ribbonlike folds are exposed to the water 

 when the shell opens ; but in the clam, the mantle 

 incloses them, forming a tube, called siphon, through 

 which the water is driven by the cilia. The aquatic 



gastropods (univalves) have 

 either tufts, like the worms, 

 or comblike ciliated gills in 

 a cavity behind the head, to 

 which the water is admitted 

 through an opening. In 

 others the breathing organ 

 is the vascular lining of this 

 cavity. The cuttlefish has 

 flat gills covered by the man- 

 tle; but the water is drawn 

 in by muscular contractions 

 of the mantle instead of by 

 cilia. The end of the siphon 

 through which it is ejected is 

 called the funnel. The gills 

 of lobsters and crabs are 



placed in cavities covered by the sides of the shell 

 (carapace) ; and the water is brought in from behind by 

 the action of a scoop-shaped process attached to one of 

 the jaws, which constantly bails the water out at the 

 front. 



The perfection of apparatus for aquatic respiration is 

 seen in fishes. The gills are comblike fringes supported 

 on four or five bony or cartilaginous arches, and contain 

 myriads of microscopic capillaries, the object being to 

 expose the venous blood in a state of minute subdivision 



FIG 275. Diagrammatic Section of a 

 Lamellibranch (Anodonta): a, lobes 

 of mantle; b, gills, showing transverse 

 partitions; c, ventricle of heart; d, 

 auricles; e, pericardium; f, g, kid- 

 neys; h, venous sinus; k, foot; A, 

 branchial, or pallial, chamber; B, 

 epibranchial chamber. 



