CRYSTALLINE STYLE OF LAMELLIBRANCHS 67 



the length of the style sac, forming a series of 'bearings' upon 

 which the style rests (figs. 9, 10). 



The typhlosoles vary in size and position according to the 

 species. In the forms examined by me they are dorsal and ven- 

 tral, and thus separate the alimentary canal into right and left 

 halves as far as the end of the style sac. In all these species I 

 have examined, the ventral typhlosole is the larger. As a re- 

 sult of the bending of the alimentary canal the typhlosole, which 

 is dorsal at the anterior end, becomes ventral throughout the 

 posterior half of the style sac. I shall, therefore, speak of the 

 typhlosoles as major and minor, rather than as dorsal and 

 ventral. 



These structures gradually diminish in size from before 

 backward. In an adult Anodonta, 14.5 cm. long, the major 

 typhlosole measures about 3 mm. across at the anterior end, 

 narrowing down to 2.5 mm. at the posterior end of the sac. 

 The minor typhlosole measures'2 mm. and 1 mm., respectively. 



In the species which have just been considered, where the 

 style caecum and intestine are completely separated, the two 

 tubes merge into one at the end of the style-bearing region. 

 Where the separation is more nearly complete, as in some of the 

 Unionidae a small diverticulum occurs at the end of the style 

 sac at the point where the intestine makes the sharp backward 



Fig. 3 Area barbata; transverse section through alimentary canal posterior 

 to the stomach (Matthias, fig. 17). epf, epithelial folds; ig, intestinal groove; 

 st, crystalline style. 



Fig. 4 Anodonta cellensis; plaster-of-Paris mold of stomach, oesophagus, and 

 anterior portion of intestine and style sac (Gutheil, fig. 3). ca, dorsal caecum; 

 int, intestine; Idi, Id,, left and dorsal ducts of hepato-pancreas; sto, stomach; 

 sts, style sac. 



Fig. 5 Dreissensia polymorpha; sagittal section through stomach, intestine, 

 and anlage of the style caecum (Meisenheimer, fig. 132). int, intestine, stc, 

 wall of somach which evaginates to form the caecum of the crystalline style; 

 sto, stomach. 



Fig. 6 Dreissensia polymorpha; longitudinal section through the style cae- 

 cum (Meisenheimer, fig. 133). dial, diatoms; gs, gastric shield; st, crystalline 

 style; stc, caecum of the crystalline style; sto, stomach. 



Fig. 7 Anodonta cellensis; transverse section through the major typhlosole 

 and style sac showing distribution of blood-vessels (Schwanecke, fig. 24 a, some- 

 what diagrammatic), av, visceral artery; sts, style sac. 



