46 MORPHOLOGY OP 



The atrial cavity must be described in this connec- 

 tion. As stated in the general account^ its origin is due to 

 the backward growth of the collar-fold to form an operculum. 

 In B. Kowalevskii {v. fig. 88) it is more marked than in 

 B. minutus, but in B. salmoneus the collar-fold does not 

 reach as far as the first gill slit^ which consequently opens 

 directly to the exterior (fig. 107). In B. Kowalevskii it 

 covers about three gill slits. (In fig. 88 only one gill-slit is 

 thus shown ; this is owing to the slight obliquity of the 

 section.) 



The relation of the opercular fold in B. minutus is shown 

 in figs. 73 and 104. 



The dorsal wall of the branchial chamber is thickened in 

 the middle line to form a ridge (figs. 89 and 92). This ridge 

 contains a groove in its posterior part. It is no doubt a sup- 

 porting structure, and may conceivably be homologous with 

 part of the backward extension of the notochord in other 

 Chordata. 



The digestive tract follows upon the branchial region. 

 The branchial chamber ends in a short blind sac above it, 

 and it is in this sac that the new gills are added after three 

 pairs are formed {v. fig. 44). The walls of the digestive tract 

 in B. Kowalevskii are thrown into an irregular spiral fold 

 {v. figs. 82 and 108), which is not continued into the intes- 

 tinal region as a definite feature. 



The cells of the digestive region are arranged (fig. 82) in a 

 single layer for the most part. They contain large granules 

 and bear a few long cilia. In the walls of the gut in this 

 region are numerous blood-vessels. The lumen of the gut in 

 this region varies greatly in size, probably with the digestive 

 processes (cf. Salensky, loc. cit.), the liver being in 

 B. Kowalevskii occasionally obliterated. 



In B. Kowalevskii there is no distinct sacculation to 

 form the liver, but in B. minutus the dorso-lateral walls of 

 the digestive region are pushed out to form the characteristic 

 liver outgrowths. These structures are not regularly paired. 

 Their walls are full of secondary foldings {v. fig. 93). The 



