CLAM 145 



tion. This blood after passing through the mantle is returned 

 directly to the heart. 



3. Respiratory System 



The chief respiratory organs of the Clam consist of two pairs 

 of gills which, as we have seen, lie in the mantle cavity on either 

 side of the visceral mass. Each gill consists of two ciliated, elon- 

 gated, perforated sheets of tissue, the lamellae, which are sepa- 

 rated a short distance from each other by regularly arranged 

 vertical rods, the interlamellar junctions. The edges of the 

 lamellae, with the exception of the dorsal surface, are fused to- 

 gether. Each gill may be compared in its structure, therefore, to a 

 very narrow, elongated bag, the top, or dorsal side, of which is 

 open. The cavity of the bag is divided by the interlamellar junc- 

 tions into a number of vertical compartments, the water tures. 



The lamellae of the gills are made up of a large number of defi- 

 nitely arranged units, the gill filaments. These run parallel to 

 the interlamellar junctions and give the gill a distinct dorso ven- 

 tral striation. The filaments are connected by interfilamental 

 junctions. Regular openings, the ostia, are situated between the 

 gill filaments so that water can pass through them and into the 

 spaces, or water tubes, between the lamellae. 



The gill filaments are covered with ciliated epithelial cells, the 

 cilia of which exhibit a coordinated, beating movement. This 

 causes a continuous current of water to be drawn into the mantle 

 cavity through the inhalent siphon. The water then flows through 

 the ostia of the gills into the water tubes, thence dorsally into a 

 longitudinal suprarranchial chamrer which opens posteriorly 

 into the exhalent siphon. During the passage of the water through 

 the gills, the interchange of carbon dioxide from the animal for 

 oxygen in the water takes place through the thin walls of the gills. 

 The cilia on the gills exhibit considerable differentiation both in 

 size and function. The beat of some of them is such as to produce 

 a current of water carrying the food particles toward the labial 

 palps which convey them to the mouth. 



4. Excretory System 



The excretory organs of the Clam consist of a pair of kidneys 

 which lie below the pericardial cavity and extend slightly beyond 

 it posteriorly, nearly to the adductor muscle. Each kidney con- 



