520 WILLIAM BATESON. 
stated that, though the origin of these structures was uncertain, 
the balance of evidence favoured the view that they were hypo- 
blastic. Since the above was written I have been led to 
regard them as more probably mesoblastic, owing to some of 
the appearances since observed. It should be noticed that the 
body cavity is continued into the valves always, but never into 
the bars separating adjacent gill-slits in which the bordering 
bars are in contact. This is due to obliteration of the cavity 
by the skeletal bars. This feature is very useful in distin- 
guishing these parts in sections. 
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. (ln 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 
