THE BNTEEOPNEUSTA. 9 



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 g. si). 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 



