CRYSTALLINE STYLE OF LAMELLIBRANCHS 87 



portion of the alimentary canal were such as to carry a secretion 

 to this point. Furthermore, I find at one side of the caecum it- 

 self a group of cells which form a structure very similar to the 

 secreting epithelium of other forms. It may be concluded, 

 therefore, that where the style exists in a separate caecum it is 

 likewise the product of an epithelial secretion. 



Embryology of the style and style-bearing organs 



Concerning the ontogenetic development of the crystalline 

 style the literature is singularly silent. Reference to it is found 

 in the work of Drew ('01) in the passage quoted above (p. 81) 

 and also in the following words. Speaking of the alimentary 

 tract of the veliger larva of Nucula, he says (p. 349) : 



The cells of the epithelial walls of the stomach are of two kinds. 

 Those at the anterior end of the* stomach carry comparatively few 

 cilia and those at the posterior end carry many ciKa. At this stage 

 some of the cells on the dorsal side of the stomach, near its anterior 

 end, begin to secrete a mucus-like material that extends posteriorly 

 in the lumen of the stomach as a small rod that probably represents 

 the cr sta line style. Later the posterior portion of the whole dorsal 

 division of the stomach .... is given over to secreting this 

 material, but a definite rod m y not be present. 



In the same year Meisenheimer described the development of 

 the crystalline style in Dreissensia, a form wdth a distinct style 

 caecum. According to this investigator (p. 93), the anlage of 

 the style caecum lies in the posterior wall of the stomach, to 

 the right of the opening of the intestine. This region is charac- 

 terized by the presence of very long bristle-like cilia (fig. 5). 

 The epithelium of the stomach wall at this point evaginates to 

 form a cone-like depression, the cells of which secrete a sub- 

 stance which is formed into a thick rod extending into the 

 stomach (fig. 6). This blind sac continues to grow posteriorly 

 until it finally forms a long caecum on the right side of the body. 

 It opens into the stomach to the right of the intestinal orifice. 

 The food of the larva of this species consists of plankton, but in 

 consequence of the presence of hard particles in the stomach, 

 incidentally acquired during feeding, Meisenheimer favors Bar- 



