THE PHYLUM MOLLUSCA 



ventrally in the mantle cavity. Here they are bound together by mucus and 

 eHminated through the incurrent siphon by periodic vigorous contractions of 

 the adductor muscles. Among the mollusks the ciliary-mucus feeding mecha- 

 nism is best developed in the pelecypods, although some sessile gastropods 

 have evolved a similar method. Certain tube-dw^elling annelids and a few 

 primitive chordates also depend on ciliary currents and sheets of mucus to 

 entrap their microscopic food. 



The digestive system of the clam consists of mouth, esophagus, stomach, 

 intestine, rectum, and anus (F'ig. 13.1). The esophagus is short and leads 

 directly into the stomach, which also receives the openings of a pair of 

 branching digestive diverticula. The stomach of many pelecypods is provided 

 with another diverticulum, the style sac, which secretes and holds in its lumen 

 a semisolid, gelatinous rod, the crystalline style. This rod is kept in constant 

 rotation by the cilia lining the sac, and its free end protrudes into the cavity 

 of the stomach, where its substance gradually and continuously dissolves. 

 This rod is essentially a mass of the secretion products of gland cells in the 

 style sac. It contains digestive enzymes, largely amylases, which are re- 

 leased into the stomach to function in the preliminary steps of the digestive 

 process. Particles of partially digested food are conducted by ciliary currents 

 into the branching lumen of the digestive diverticula, where digestion is con- 

 tinued. Finally, particles of food are engulfed by cells lining the passages 

 within the diverticula, and the process of digestion is completed intracellularly. 

 Absorption occurs in the diverticula, and to some extent in the anterior parts 

 of the intestine. The major function of the relatively long intestine and 

 rectum appears to involve the dehydration and concentration of digestive 

 wastes into fecal pellets or strands, which are eliminated at the anus into the 

 outflowing streams of water in the cloaca. 



The well-developed circulatory system constitutes what might be called an 

 "open" system. A pulsatile heart is present, consisting of a pair of lateral 

 atria (auricles) and a thick-walled, tubular ventricle (Fig. 13.2). The ventricle 

 is wrapped around the rectum, but this fact appears to have no functional 

 significance. From the ventricle spring an anterior and a posterior aorta. 

 These are the chief distributing vessels which carry blood through specific 

 branches to the various organs of the body, including the foot, the mantle, 

 the gills, the excretory organs, the alimentary canal, and so on. Within these 

 organs the blood flows through spaces which are not lined by a continuous 

 endothelium, and thus are sinuses rather than capillaries. In such an "open" 

 system the nutrients and oxygen carried in the blood may diff^use directly into 

 the intercellular fluid, without passing through the walls of blood vessels. 

 This would appear advantageous; but, in comparison with capillary systems, 

 the efficiency of a system of sinuses is reduced by the fact that blood pressure 

 is necessarily low, and the velocity of flow is likewise diminished. After its 

 circuit through the tissues, the blood is returned by collecting vessels into 

 the thin-walled atria and thence into the ventricle. The course of this 

 circulation is diagrammatically presented in Figure 13.3. 



373 



