136 Invertebrate Zoology. 



Internal Anatomy. Viewing the specimen from the 

 left side, the course of the S-shaped digestive tract may 

 readily be followed. The tract is made up of two parallel 

 portions, the oesophagus and stomach forming the lower 

 limb, and the intestine forming the upper. Lying just 

 above the, often dark-colored, intestinal loop is the light- 

 colored sexual gland. The delicate sexual duct extends 

 dorsally and posteriorly, parallel with the posterior section 

 of the intestine. 



Lying deeper than the just mentioned viscera are six 

 nearly parallel light-colored bands. They are the branchial 

 folds, and will be considered later on. 



At a point on the dorsal surface, about midway between 

 the siphons, the ganglion will be observed as a small opaque 

 spot. Notice the nerves, and, if possible, trace them to the 

 siphon tubes. 



Again follow the course of the opaque endostyle. It ex- 

 tends along the mid-ventral line, from near the base of the 

 oral siphon to a point removed nearly 180. Its posterior 

 termination is near the point of origin of the oesophagus. 1 



Through the transparent mantle of the right side six 

 branchial folds and a large sexual gland will be at once 

 recognized. The duct leading from the sexual gland 

 extends in a nearly horizontal direction towards the cloacal 

 siphon. It often contains eggs or embryos. Lying parallel 

 to and below the sexual gland, enclosed in a pericardium, is 

 the heart. (In alcoholic specimens the circulatory system 

 is not easily dissected.) Below (ventral to) the heart is a 

 crescentic sac of considerable size. It is the renal organ, 

 and often contains brownish flakes and concretions. 



1 The endostyle may be the homologue of the hypobranchial groove 

 of Amphioxus, and of the thyroid gland of higher Vertebrates. 



