78 



GENERAL DEVELOPMENT. 



terior section corresponds to the region of the oesophagus and 

 stomach, but exhibits no distinction 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 con- 

 nected in front with a well-developed pronephros (head-kidney), with 

 about five ciliated funnels opening into the pericardial 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 Amphioxus. It is soon able to swim 

 with vigour, but usually, unless disturbed, is during the day quite 

 quiescent, and chooses by preference the darkest situations. It soon 

 straightens out, and, with the disappearance of the yolk, the tail 



Fio. 47. Hkad of a lakva of Petkomyzon six weeks old. 

 (Altered from Max Schultze.) 

 au.v. auditory vesicle; op. optic vesicle ; ol. olfactory pit ; ul. upper lip; II. lower 

 lip; or. J), papillae at side of month; v. velum; hr.s. extra brancliial skeleton; 1 — 7. bran- 

 chial clefts. 



becomes narrower than the head. A large caudal fin becomes 

 developed. 



When the larva is about twenty days old, it bears in most ana- 

 tomical features a close resemblance to an Ammocoetes ; though the 

 histological differences between" my oldest larva {29 days) and even 

 very young Ammocoetes are considerable. 



The mouth undergoes important changes. The upper lip becomes much 

 more prominent, forming of itself the anterior end of the body (fig. 47, ul). 



