ALIMENTARY CANAL. 139 



In Chimaera a small portion of the supraorbital canal is supplied by 

 the ophthalmicus profundus, and in some if not all Plagiostomes {Mus- 

 telus, Laemargus), Teleosts, Ganoids, the dorsal branch of the glosso- 

 pharyngeal innervates a small portion of the cephalic lateral line. 



For an account of the sense organs the reader is referred 

 to p. 121, and to the section deahng with Pisces (pp. 77-82). 

 For the sympathetic, see p. 75. 



The alimentary canal is fairty similar throughout the order. 

 The mouth is usually ventral. Teeth varying considerably in 

 shape are present in several rows on the palato-quadrate bar and 

 cartilage of Meckel. As those in the row next the mouth 

 opening are worn away, those of the next row advance, and a 

 new row is added internally. The basihyal projects in a tongue- 

 like manner from the floor of the mouth. There are no salivary 

 glands. The hinder part of the mouth passes without demarca- 

 tion into the pharynx which receives the internal openings of 

 the spiracle and gill-slits. 



The oesophagus leads into the stomach, which is U-shaped, 

 the pyloric limb of the U being narrow and opening into the short 

 somewhat swollen anterior end of the intestine (duodenum, bursa 

 entiana). Into this open the bile duct and the pancreatic duct. 

 It is followed by the rest of the intestine which is provided with 

 a spirally disposed longitudinal valve. The intestine ends behind 

 in a narrow rectum, which receives a dorsal gland, the rectal 

 gland, and opens into the cloaca. The cloaca opens externally 

 between the pelvic fins. The alimentary canal is supported by 

 a mesentery which is defective in part. The liver is well devel- 

 oped and usually provided with a gall bladder. The pancreas 

 is also large. There is a well-developed spleen in the neighbour- 

 hood of the stomach. 



In Laemargus borealis two large caeca open into the commencement 

 of the intestine. In some forms (Zijgaena, Carcharias) the longitudinal 

 valve is not spirally arranged, but is rolled upon itself. 



There is no air-bladder. 



A thyroid is present between the rami of the mandibles. It 

 frequently has a pyriform shape and lies over the bifurcation 

 of the ventral aorta. 



The thymus is represented by an elongated lobed gland placed 

 over the dorsal ends of the branchial arches and derived, as usual, 



