HABITS AND STEUCTURE OF ARENICOLA MARINA. 13 



The oesophagus^ itself is slightly looped behind the second 

 diaphragm. It is a thin-walled distensible tube^ the first part 

 of which is lined by non-ciliated mucus-forming cells. The 

 middle portion is lined by a cuticle, and the posterior part by 

 cells resembling those of the stomach in bearing cilia. The 

 oesophageal pouches {Oe. Gl.) are somewhat flask-shaped, and 

 open into the cavity of the oesophagus by a short tubular 

 stalk. They are usually greenish in colour, but have a slight 

 reddish tinge on account of their very large blood-supply. 

 Their blood-vessels are connected with the lateral oesophageal 

 and dorsal vessels. The cavity of the pouch is subdivided by 

 twenty-five to thirty incomplete partitions, produced by in- 

 folding of the wall of the pouch, and therefore covered on each 

 side by the epithelial lining of the pouch (PI. 4, fig. 22). 

 Between the epithelial lamellae is a blood-sinus, which is 

 slightly enlarged at the inner end and slightly thickened at the 

 edge of each partition. The oesophageal pouches are lined by 

 ciliated epithelium, covered with a fairly stout cuticle, and 

 contain glandular cells. The walls of the oesophagus are 

 marked by longitudinal and circular muscular impressions. 



The stomach, marked out by the patches of yellow tissue 

 on its walls, extends from the level of the heart to about the 

 twelfth notopodial setse. As we have already stated (p. 9), 

 the stomach is bent upon itself and loosely attached to the 

 body-wall. The patches of ^"^ chlorogogenous " tissue are at 

 first arranged in symmetrical oval areas right and left of the 

 dorsal blood-vessel, while more veutrally they are placed in 

 two or three less regular series, and are separated from one 

 another by a network of blood-vessels.^ About the level of 

 the tenth setse these yellow areas all become subequal and 

 arranged in a spiral manner, ending at the level of the four- 

 teenth setae. 



Stomach and Intestine. — The muscular wall of the 



1 Tlie histology of the alimentary canal has been carefully investigated by 

 Wiren (1887, p. 31). Our results agree very closely with his. 



^ This network is considered by Wiren and others to be parts of a con- 

 tinuous sinus. We are not convinced, however, that this is really the case, 

 and our reasons will be found on p. 17 infra. 



