DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 89 
skeleton, &c., will be given when the histology of the gills 
is described. 
Together with the increase in number of the gills the differ- 
entiation between the digestive and intestinal region becomes 
more prominent, the bright yellow-brown of the former show- 
ing through the transparent body wall, being a most striking 
feature in the appearance of the animal. When first per- 
ceptible, the digestive tract is a simple tube, separated by a 
slight constriction from the intestine. As growth proceeds 
this constriction becomes more marked, and when the second 
gill-slit is fully formed the separation between the two is 
sharply defined (fig. 2). Subsequently a fold arises in the 
digestive region which gives it the appearance of being made 
up of two saccules (two gill-slits). This condition becomes 
more and more marked, and then a third saccule appears 
posteriorly (4—5, gs.). When, however, the animal is seen 
from the dorsal side the alimentary canal is seen to have a 
wavy contour, the two saccules being thus parts of a slightly 
bent tube. Moreover, in longitudinal sections the divisions 
between the saccules are parts of a spiral fold which traverses 
the whole digestive region. When five gill-slits are formed 
there are three saccules, and in animals with ten gill-slits they 
are five in number. After this the body walls become much 
more opaque, attaining the condition which they present 
throughout adult life. It is consequently not possible to follow 
the internal development after this stage by means of surface 
views. 
The walls of the intestinal region are also thrown into folds, 
but their arrangement would appear to be irregular. The cells 
in this tract bear long flagelliform cilia which appear to drive 
a current through the anus. 
The anus is first found at about the time of the formation of 
the second gill-slit. As previously mentioned it is almost if 
not quite in the position in which the blastopore closed, being. 
posterior, median, and dorsal. When the tail is formed the 
anus is immediately dorsal to it, in fact, its ventral margin is 
formed by the dorsal side of the tail. In ordinary conditions 
