THE ELASMOBRANCH FISHES 



123 



covery tliat in the young embryo of H. cinercus the thymus is not a ductless 

 gland. In this type ducts lead from the first six lobes of the thymus into the 

 pharynx. The thyroid gland is located at the symphysis of the lower jaws 

 between the coracomandibularis and coracohyoideus muscles (see p. 200, fig. 

 184, th.). It is a mass of semitransparent glandular tissue slightly crescentic 

 in shape and surrounded by a capsule of connective tissue. 



Oesophagus 



The digestive tract is continued posteriorly from the pharynx by a short, 

 thick-walled portion, the oesophagus (oe., fig. 120) . The lining of the oesopha- 

 gus is thrown into whitish longitudinal 

 folds, some of which may be continuous 

 with those of the stomach. The oesopha- 

 gus is distinguished from the stomach, 

 however, by the character of its folds, 

 the folds of the latter being much more 

 pronounced. The epithelium of the 

 oesophagus differs from that of the 

 pharynx in that it does not contain 

 stomodeal denticles. 



Stomach 



Fig. 122. Section through thymus gland 

 of 63 mm. embryo Heptanchus cinereus. 

 (From Van Wijhe.) 



d., duct of thymus; e., epithelial body; 

 lim., hyomandibular cartilage; oi., otic 

 area; p-q., palatoquadrate cartilage; //(., 

 thymus nodules; //, second gill cleft. 



The stomach of Heptanchus is U- 

 shaped, the larger left limb being the 

 cardiac end (f..s\, fig. 120), and the 

 smaller right limb, the pyloric division 

 (p.s.). Superficially the cardiac por- 

 tion of the stomach appears as a more 

 or less distended bag. while the pyloric 



division is thick-walled. Within the cardiac stomach is usually found a consid- 

 erable amount of undigested food material in the form of pieces of fishes, the 

 shell and claws of crabs, and the like. 



Internally the pronounced folds of the cardiac stomach continue in a more 

 or less longitudinal direction from its union with the oesophagus throughout 

 two-thirds of its length where they become more or less tortuous. At the distal 

 end of this part of the stomach there is present a blind sac in which some of 

 the folds terminate. Some of the small folds abut against a circular fold which 

 in a way separates the cardiac from the pyloric division. The pyloric lumen 

 of the stomach is long and narrow, and its folds are not especially marked at 

 the proximal end. As the terminus of the pylorus is reached, however, they 

 become much higher. Three-fourths of an inch from the termination of the 

 pyloric linil) there is a slightly enlarged portion which opens into the duod- 

 enum through the pyloric valve. 



