hindgut; this is lacking in the teleost, at least in the adult. 

 The midgut region is much elongated in the ray-finned 

 fishes, with the exception of Polypterus, and is sometimes sep- 

 arated from the hindgut by a valve. 



The selachian and holocephalan resembles higher verte- 

 brates in having a compact pancreas, while the actinopteryg- 

 ian is peculiar in having a diflfuse pancreas and pyloric 

 caeca. Polypterus agrees with the actinopterygian in this. In 

 the dipnoan, the pancreas is concealed within the gut wall, 

 as is the spleen, but both are separate compact structures in 

 the actinistian. The lungfish and actinopterygian appear to 

 be modifications away from an original condition. 



Cyclostomes 



Lamprey The mouth of the lamprey (Figure 9-26) is a 

 circular opening set deep in a funnel-like sucker, armed 

 with rows of horny teeth, and fringed with numerous fleshy 

 papillae. The mouth opening itself is blocked by a tooth- 

 bearing, rasping organ often referred to as the tongue. This 

 tongue is an elongated muscular structure lying in the floor 



stomodeum 



proctodeum 



7.5 mm 



neurenteric canal 



stomach pancreas spiral valve 



mtestme 



mesonephric duct 



liver diverticulum 

 B 20.6 mm 



spleen 



piral valve 

 Intestine 



L-i J' X I ^ rectal gland 



bile duct I ' ^ 



pancreas 



interriol yolk sac (lies on right side) 

 external yolk sac 



C 110 mm 

 Figure 9-23. Three stages in the development of the gut and yolk 

 sac of the shark, Squalus aconthias. (After Nelsen, I 953) 



of the oral or buccal cavity and extending back below the 

 branchial region. The tongue moves forward and back in 

 helping create the suction by which the animal attaches to 

 its prey or to rocks when moving upstream against a strong 

 current Other muscles move the teeth on the tip of the 

 tongue up and down giving them a rasping action which 

 cuts into the flesh of the prey. A pair of sac-like salivary 

 glands, embedded in the tongue, open into the oral cavity 

 at the base of the rasping teeth. These glands produce a 

 secretion which prevents coagulation of the blood of the 

 prey. 



Behind the buccal cavity, the pharynx is divided into a 

 dorsal tubular pharyngoesophagus and a ventral respiratory 

 passage. On the anterior margin of this dividing septum, 

 there are several forward-projecting tentacles supported by 

 the cartilaginous rods of the velar skeleton. Below this ten- 

 tacular margin and projecting posteriorly, is a flap of tissue 

 on either side which serves as a valve. This valve eliminates 

 backflow from the respiratory pharynx. In the larva the 

 pharynx is undivided, but at metamorphosis the separation 

 of the cavity occurs from behind forward. This process in- 

 volves development of the rasping apparatus as well. The 

 velar tentacles mark the position of the septum between the 

 stomodeum and pharynx of the embryo. 



The pharyngoesophagus is constricted posteriorly to a 

 narrow opening, then widens out to open on the left through 

 a longitudinal slit into the gut. This slit is extended by flaps 

 on either side well into the gut cavity. This slit extends pos- 

 teriorly into the anterior end of the typhlosole. Well behind 

 this entrance the bile duct opens in a deep groove to the left, 

 or above, the typhlosole. 



The anterior end of the gut is expanded, while posteriorly 

 it is of varying but reduced diameter. Most lampreys used 

 in the laboratory are collected on their way to spawn at a 

 time when they do not feed, so the gut is small. The gut lies 

 free for much of its length in the body cavity. Anteriorly it is 

 attached, right dorsolaterally, to the liver along a line extend- 

 ing much of the length of the expanded anterior part. The 

 posterior fifth of the gut is attached dorsally by two or three 

 vascular connections wdth the dorsal body wall. The anus 

 opens into a cloacal slit in which lies the urogenital papilla. 

 The liver is a conical mass, hollowed above and rounded 

 below. Anteriorly its base is attached all around to the body 

 wall, and posteriorly it tapers to a rounded point below the 

 gut. The gall bladder lies within the substance of the liver 

 anteriorly at the midline; it is not conspicuous in sexually 

 mature adults The bile ducts join in the region behind the 

 bladder and the main duct passes upward and medially to 

 enter the gut near the posterior end of its anterior swelhng. 

 Behind and above the entrance of the bile duct into the 

 gut is an islet of Langerhans mass. A second large islet mass 

 occurs above the constriction of the pharyngoesophagus at 

 its opening into the gut. A third mass occurs midway be- 

 tween these. In the wall of the anterior part of the intestine 

 are large patches of cells which resemble those of the pan- 



THE DIGESTIVE TRACT AND ASSOCIATED STRUCTURES • 275 



