46 LABORATORY DIRECTIONS 



will now be in view. Begin at the corner of the mouth and cut 

 back through the gill arches on the left side until the body cavity 

 is reached. Find the following: 



1. Pharynx, the region in the back part of the mouth cavity 

 common to both digestive and respiratory systems. Lay back 

 the floor of the mouth and study the gills and the gill arches. 

 The gills exist as rows of delicate gill filaments radiating from the 

 membrane covering the gill arches. The first arch, situated on 

 the anterior wall of the first gill slit, is the hyoid arch. It is 

 termed a hemibranch as it bears gill filaments on the posterior 

 side only. A holobra?ich has gill filaments on both sides of the 

 arch. How many holobranchs and hemibranchs are there and 

 to which arches do they belong ? The hyoid arch, in addition to 

 bearing gill filaments, supports the tongue. The small carti- 

 laginous processes on the inner borders of the gill arches are the 

 gill rakers. 



The spiracles, bearing gill filaments and opening into the 

 pharynx, have already been studied. 



2. Oesophagus, an exceedingly short tube connecting the 

 pharynx with the stomach. 



3. Stomach, the large J-shaped organ extending more than 

 half the length of the body cavity. It may be partly covered 

 by the liver. The size of the stomach will depend to a large 

 extent on the amount of food it contains. It is commonly con- 

 stricted near the middle. Slit open the stomach, wash out 

 anything it may contain, and examine the inner surface. The 

 anterior part contains finger-like papillae; further back are 

 longitudinal folds, which vary in size with the degree of con- 

 traction of the stomach. In a stomach distended with food the 

 longitudinal folds may wholly disappear. A pyloric valve is 

 present on the caudal end of the stomach. 



4. Intestine, extending from stomach to anus, and consisting 

 of three regions. The first part, the duodenu7n, is short; it lies 

 between the stomach and the second region, the large spiral 

 valve. The spiral valve constricts posteriorly to form the short 

 third division of the intestine, the rectum, which opens into the 

 cloaca. Attached to and opening into the rectum is a finger-like 

 projection, the rectal gland, the function of which is not known. 



