1 8 ALIMENTARY TRACT. 



Kowalevsky states that a pore is formed at the front end of the nervous 

 tube leading into the mouth (fig. 8 v. and vi. /) which eventually gives rise 

 to the ciliated sack, which lies in the adult at the junction between the mouth 

 and the branchial sack. Kupffer however was unable to find this opening ; 

 but Kowalevsky's observations are confirmed by those of Salensky on 

 Salpa. 



From the hinder end of the alimentary sack an outgrowth 

 directed dorsalwards makes its appearance (figs. 8 V. and 9, d), 

 from which the oesophagus, stomach and intestine become 

 developed. It at first ends blindly. The remainder of the 

 primitive alimentary sack gives rise to the branchial sack of the 

 adult. Just after the larva has become hatched, the outgrowth 

 to form the stomach and oesophagus, etc. bends ventralwards 

 and to the right, and then turns again in a dorsal and left 

 direction till it comes close to the dorsal surface, somewhat to 

 the left of and close to the hinder end of the trunk. The first 

 ventral loop of this part gives rise to the oesophagus, which 

 opens into the stomach ; from this again the dorsally directed 

 intestine passes off. 



On the ventral wall of the branchial sack there is formed a 

 narrow fold with thickened walls, which forms the endostyle. 

 It ends anteriorly at the stomodaeum and posteriorly at the 

 point where the solid remnant of the archenteron in the tail was 

 primitively continuous with the branchial sack. The whole of the 

 alimentary wall is formed of a single layer of hypoblast cells. 



A most important organ connected with the alimentary 

 system still remains to be dealt with, viz. the atrial or peri- 

 branchial cavity. The first rudiments of it appear at about the 

 time of hatching, in the form of a pair of dorsal epiblastic 

 involutions (fig. 8 v. /CV), at the level of the junction between the 

 brain and the spinal cord. These involutions grow inwards, and 

 meet corresponding outgrowths of the branchial sack, with 

 which they fuse. At the junction between them is formed an 

 elongated ciliated slit, leading from the branchial sack into the 

 atrial cavity of each side. The slits so formed are the first pair 

 of branchial clefts. Behind the first pair of branchial clefts a 

 second pair is formed during larval life by a second outgrowth 

 of the branchial sack meeting the epiblastic atrial involutions 

 (fig. 8 VI. \ks and 2/fo). The intestine at first ends blindly close 



