402 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



The stomach. — This organ is a greater or less enlargement of the alimentary canal, 

 and is placed in the dorsal part of the visceral mass, its long axis generally being that 

 of the body. In Yoldia, however, it is dorso-veutral (Fig. 09). In many cases, as in 

 Venus, it is close to the dorsal wall of the visceral mass (Fig, 12, s). Frequently in 

 individual cases it lies much more in one side of the visceral mass than in the other. 

 The walls are often irregular (Figs. 12, 26, 44, s). Closely applied to the stomach 

 walls is the liver mass, whose ducts open into the stomach at different points. The 

 number of these openings varies in different forms. 



The intestine. — The stomach generally narrows posteriorly to open into the intes- 

 tine {i in the figures). In most cases this is coiled. In the degenerate deep-sea forms 

 recently studied by Pelseneer (No. 17), this observer describes the intestine as being 

 so short as to be almost straight. He supposes that this condition has been brought 

 about by the carnivorous habit of the animals, which is inferred fi-om the animal mat- 

 ter found in the digestive tract. 



The intestine is but little coiled in Yoldia. Fig. 69, PL lxxxviii, represents the 

 whole tract in this form. The mouth (m), short oesophagus, and dorso-ventrally elon- 

 gated stomach (s), are seen anteriorly. From the bottom of the stomach springs the 

 intestine (i). This proceeds backward, then upward as far as the top of the stomach. 

 With a bend forward, it makes a looi5, always on the right side of the stomach, before 

 running backward over the i)Osterior adductor {j)a) to empty as the rectum (r) into 

 the cloacal siphon. 



In other lamellibranchs the intestine is generally much more convoluted. In the 

 oyster, for example, it runs downward and backward from the stomach so far as to be 

 ventral to the adductor nuiscle. It returns to the upper part of the stomach, makes a 

 comi>lete looj) around it, and then proceeds back over the i^ericardium aud adductor 

 muscle. The extreme end of the rectum hangs close to the base of the cloacal siphon 

 in forms where it is present (Figs. 93, 94, r) ; in otlier cases it projects well into the 

 cloacal chamber (Figs 89, 97, r). The extreme end is often slightly thickened. 



Crystalline style. — This organ is a diverticulum of the stomach or the intestine, 

 generally close to the latter. In the diverticulum, where it is much developed, is the 

 transparent style, evidently a product of the secretion of the epithelium of the diver- 

 ticulum. Pelseneer says that the stomachs of all lamellibranchs are lined with a 

 cuticular covering which is continuous with the style when the latter is i^resent. 



Fig. 26, PI. Lxxxiii, reijresents the greatly developed style of Mya arenaria {est) 

 arising fi'om the bottom of the posterior end of the stomach [s) and running ventrally 

 with a bend to the right, so nearly in a vertical plane as to be shown all the way in 

 the section. Reaching the ventral part of the visceral mass, it runs forward a short 

 distance (Fig. 25, est) and ends blindly. In Anomia it even runs out of the visceral 

 mass into the mantle edge. 



The style may be easily drawn out of the sheath-like diverticulum. In cross- 

 section, it appears to consist of innumerable concentric lamelloe. If the large style 

 of Mactra be taken out of the animal and an end of it carefLilly teased, we find that it 

 seems to have a central, apparently softer axis, around which the lamellae are depos- 

 ited in a concentric spiral. When an end of any width is artificially made, as repre- 

 sented in Fig. 55, PI. Lxxxvii, this outer portion may be unwound to any extent from 

 the central axis, showing its spiral arrangement. 



