CRYSTALLINE STYLE OF LAMELLIBRANCHS 65 



2. ORIGIN 



Gross structure of the style-bearing organs 



The crystalline style and its attendant structures in Anodonta 

 have been described, p. 56. The condition found in the other 

 groups of the lamellibranchs follows: 



The style occurs in its simplest form in the more primitive 

 groups, the Protobranchs and Filibranchs. In Nucula, as an 

 example, the style lies in the intestine itself, the alimentary 

 canal showing no indication of a separation into two tubes. In 

 the Filibranchs are certain transitional forms. Area, for in- 

 stance, contains species which, according to Matthias, show the 

 simple condition, as in Nucula, and others in which there is the 

 beginning of the formation of a separate style sac. 



In Area platei the intestine is roughly ovoid in cross section 

 and is nearly filled by the style. At one side the intestinal wall 

 , forms a slight groove down which pass the waste materials. 

 In Area barbata this groove is somewhat deepened and, by the 

 infolding of the intestinal wall, forming two ridges, this furrow is 

 partially separated from the part bearing the style. These in- 

 foldings, which represent the typhlosoles of Anodonta in their 

 most primitive condition, are not in apposition in this species, 

 but leave a wide space through which the intestinal groove and 

 the style-bearing portion are in communication (fig. 3). 



A condition intermediate between that found in these two 

 species occurs in Modiolaria marmorata where, according to 

 List ('02) only a very slight infolding of the intestinal wall 

 occurs. The epithelium of the alimentary tract is covered with 

 large and powerful cilia, by means of which, on the one hand, the 

 style is pushed forward into the stomach, and on the other, the 

 waste matter is passed outward. By continued growth and dif- 

 ferentiation, the epithelial infoldings of the intestine develop 

 broad flat surfaces which come into apposition and separate the 

 intestine from the style sac save for a narrow slit, as found in 

 Anodonta (fig. 2). Coincident with these changes to form the 

 typhlosoles, the epithelium of the style-bearing region becomes 

 thrown up into a series of transverse folds, extending throughout 



JOURNAL OFMORPHOLOGV, VOL. 31, NO. 1 



