DEVELOPMENT OF BALANOGLOSSUS KOWALEVSEII. 87 
(1) Disappearance of ciliated band and apical tuft. 
(2) Body walls have become semi-transparent. 
(3) Animal has changed its habitat, creeping into the upper 
layer of mud. 
(4) A fixing organ has formed as a ventral posterior sucker. 
(5) Owing to growth of collar-fold, and constriction of pro- 
boscis-stalk, the mouth comes to be directed forwards. 
(6) Differentiation has occurred in the hypoblast cells, 
marking out the alimentary canal into three regions, VIZ. 
branchial, digestive, and intestinal. 
(7) Notochord is distinctly visible. 
(8) Anal perforation present. 
(9) Second pair of gill-slits arise. 
(10) Opercular fold forms, as a circular thickening. 
From this point onwards the principal changes in external 
features consist chiefly in an increase in size, and in continual 
progress towards transparency. This latter is so marked a 
feature that when three pairs of gill-slits are formed the walls 
of the body behind the collar, which were originally opaque, 
become perfectly clear and glassy, so that the internal struc- 
tures are quite distinguishable. The walls of the proboscis 
and. of the collar never entirely lose their primitive opacity. 
An attempt is made in figs. 1, 2, and 3 to show this gradual 
transition. It is presumably due to the consumption of the 
food particles which in the earlier condition were distributed 
almost uniformly among the cells of the body. This great 
transparency is not usual among forms that do not lead a free- 
swimming existence, and is possibly, so to speak, accidental, 
and correlated to the rapid growth which now occurs. Nume- 
rous glandular patches and spots are to be seen in the skin. 
They are developed chiefly ou the proboscis, collar, and sucker, 
those on the latter being refractive and differing somewhat in 
appearance from the rest. 
The increase in size seems to be rapid, but as to the length 
of time that is taken in passing through subsequent stages I 
have no record, as the specimens were caught from time to 
time, and not reared in aquaria. 
