REFOKMATION OF THE CRYSTALLINE STYLE 271 



sential features, to be similar to the union of the intestine and the 

 style sac existing throughout the entire length of the latter in 

 Cardium corbis, Saxidomus giganteus, Paphya staminea, and 

 others. In these cross-sections prominent t^^Dhlosoles are ob- 

 served to be developed, one on either side, separating the intesti- 

 nal tube from the style sac (figs. 7 and 8). Here the intestine 

 is on the anterior surface of the style sac, and I have designated 

 one typhlosole as the left and the opposite one the right. 



After being in union for a short distance below the floor of 

 stomach, the intestine breaks away from the anterior border of 

 the style sac, making a sharp turn toward the stomach wall, 

 bending ventrally again and beginning the characteristic series of 

 loops in the visceral mass (figs. 1 and 17). The intestine crosses 

 to the right of the crystalline style as it ascends toward the dorsal 

 border to traverse the pericardial cavity. 



At the point of separation of the intestine from the style sac, 

 and as a result of this separation, an evagination of the style sac 

 occurs (fig. 9) . The groove thus formed continues on the antero- 

 lateral, border of the tube, becoming less prominent toward the 

 distal end, and fades away as a distinct groove among the 

 numerous folds of the inner wall at this extremity. 



Transverse sections at the point of separation of the two tubes 

 clearly indicate that the groove is a remnant of the intestinal 

 tract formed by the latter as it is drawn away from the style sac. 

 In the formation of this groove in Mya arenaria the evidence 

 points toward this species being a transitional form between 

 lamellibranchs in which the style sac is united throughout its 

 length with the intestinal tract, and those species in which the 

 intestine and the style sac are completely separated. The cells 

 lining the groove have the same general characteristics as the 

 cells of the epithelium of the intestinal tube. In the bottom of 

 the groove thus formed the cells are short, becoming longer on 

 the sides where they merge into the epithelium of the style sac 

 proper. The position and arrangement of the nuclei of cells of 

 the groove vary somewhat, due to modification cf the cells 

 in the different regions of it, but the nuclear arrangement char- 

 acteristic of the epithelium of the digestive tube is generally 



