CYCLOSTOMATA. 



95 



largely swollen with yolk. The anterior section corresponds to 

 the region of the oesophagus and stomach, but exhibits no dis- 

 tinction of parts. Immediately behind this point the alimentary 

 canal dilates considerably, and on the ventral side is placed the 

 opening of a single large sack, which forms the commencement 

 of the liver. The walls of the hepatic sack are posteriorly united 

 to the yolk-cells. At the region where the hepatic sack opens 

 into the alimentary tract the latter dilates considerably. 



The posterior part of the alimentary tract still constitutes a 

 kind of yolk-sack, the ventral wall being enormously thick and 

 formed of several layers of yolk-cells. The dorsal wall is very 

 thin. 



The excretory system is composed of two segmental ducts, 

 each connected in front with a well-developed pronephros (head- 

 kidney), with about five ciliated funnels opening into the peri- 

 cardial region of the body cavity. The segmental ducts in the 

 larvae open behind into the cloacal section of the alimentary 

 tract. 



The development of the larva takes place with considerable 

 rapidity. The yolk becomes absorbed and the larva becomes 

 accordingly more transparent. It generally lies upon its side, 

 and resembles in general appearance and habit a minute Am- 



OJJ ,, 



it //.I' 



FIG. 47. HEAD OF A LARVA OF PETROMYZON six WEEKS OLD. 

 (Altered from Max Schultze.) 



au.v. auditory vesicle ; op. optic vesicle ; ol. olfactory pit ; nl. upper lip ; //. lower 

 lip ; or.p. papillae at side of mouth ; v. velum ; br.s. extra branchial skeleton ; i 7. 

 branchial clefts. 



phioxus. It is soon able to swim with vigour, but usually, unless 

 disturbed, is during the day quite quiescent, and chooses by 



