258 



THE RISE OF ANIMAL LIFE 



horizontal rows of ciliated pores, or ostia, 

 perforate the lamellae through which water 

 enters the gill. The water tubes lead to a 

 dorsally situated supra-branchial chamber 

 that continues to the posterior portion of the 

 gill and opens into the excuiTent siphon. 

 Blood from the veins circulate through tiny 

 vessels within the gill to be aerated before 

 returning to the heart. In this manner the 

 constant stream of water flowing through 

 the gills supplies the animal with oxygen. 

 The beating cilia of the gills and the 



carried through the short esophagus to the 

 dorsally located, sac-like stomach. A pair 

 of digestive glands joins the stomach 

 through ducts. Digestion occurs both in 

 the stomach and in the glands themselves. 

 In some species of clams the crystalline 

 style, a gelatinous rod resembling a pouch 

 or caecum of the stomach, secretes a starch- 

 digesting enzyme. The intestine, leading 

 from the stomach, coils several times 

 through the visceral mass, much of which 

 is the yellow-colored, branched gonad, be- 



Fig. 12-8. Schematic drawing of the circulatory system of the clam. 



mantle draw water and food into the man- 

 tle cavity through the incurrent siphon. The 

 siphon opening serves to strain out all but 

 very minute food particles such as algae, 

 Protozoa, and bits of debris. Mucus se- 

 creted by the gills catches these particles 

 which are borne anteriorly by cilia to two 

 pairs of triangular, ciliated labial palps. 

 Here a separation takes place (Fig. 12-6), 

 the edible particles being carried into a 

 groove between the palps and then to the 

 mouth, and the debris passing out through 

 the excurrent siphon. In the buccal cavity 

 more mucus is secreted and the food is 



fore it turns dorsally to pass through the 

 pericardial cavity and the heart. Absorp- 

 tion takes place throughout the length of 

 the intestine, particularly in the portion of 

 the rectum which passes through the ven- 

 tricle of the heart. The typhlosole is a 

 longitudinal fold in the rectum, a structure 

 very similar to that found in the intestine of 

 the earthworm. Posteriorly, the intestine 

 opens through the anus, located within the 

 excurrent siphon, where the feces are 

 carried away with the out-going current of 

 water (Fig. 12-6). 



The heart, lying in the dorsal pericardial 



