1924 1 Lazier: Morphology of Digestive Tract of Teredo navdlis 463 



sac for the style opening to the stomach near the caecal orifice. Siger- 

 foos' (1908) very brief description of the crystalline style and its 

 attendant structures agrees with the present observations, though he 

 does not mention the gastric shield. Barrois (1889-1890) in his 

 classic paper on the crystalline style does not mention Teredo. 



Nelson (1918) reviews all previous investigations of this peculiar 

 molluscan structure and contributes much original material. His 

 conclusions as to its structure and function seem to end a long contro- 

 versy. The observations we have recorded here agree in all essentials 

 with his descriptions of species in which the sac is entirely separated 

 from the intestine. He did not examine any Teredo and makes only 

 one or two references to their style. He mentions a style of similar 

 shape, but of much greater proportions, in two other borers. Pholas, 

 and Meirtesia. 



Nelson (1918) found that the style is slowly rotated by the cilia 

 lining its sac and that it is gradually dissolved away. It thus helps to 

 keep in motion the contents of the stomach, and the amylolitic fer- 

 ments which it contains are set free to aid in digestion. The style in 

 Teredo doubtless functions in a similar manner. 



In Teredo navalis the crystalline style seems small in proportion 

 to the stomach, as compared with such forms as Martesia. Mya, and 

 Donax. This suggests that it is concerned only with the digestion of 

 plankton and that the ingested wood is cared for by other means. 



From the left antero-dorsal wall of the stomach projects a small, 

 pointed diverticulum, the dorsal caecum (d. c., pi. 23, figs. 6, 7). This 

 is called the secondary caecum by Sigerfoos (1908), but it is probably 

 homologous with what Gutheil (1912) calls "Magenfalte" of Ano- 

 donta. Nelson (1918) refers to this as the dorsal caecum ami this 

 seems to be the most appropriate term. This caecum is ciliated 

 internally. 



Between the dorsal caecum and the orifice of the sac of the crystal- 

 line style is a distinct outpocketing of the wall, which we have referred 

 to as the lateral pouch (I. p., pi. 23, figs. 6, 7). It is overhung by a 

 peculiar roll of epithelium. 



In lamellibranchs there is typically a typhlosole in the intestine, 

 extending more or less throughout its length, and, as a rule, anteriorly 

 into the stomach. In Teredo navalis this typhlosole, which we shall 

 call the gastro-intestmal typhlosole (g. i. t.), originates at the hepatic 

 orifice in the lateral pouch (pi. 23, fig. 6), passes across the stomach 

 below the opening of the oesophagus and posteriorly along the right 



