PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 235 
thoracic appendages; the abdominal segments are flexed ventrally, and 
the thoracic endopods, hanging down, steady the rolling motions some- 
what. In the second stage the conditions are essentially the same. 
In the third stage the larve are more stable, though the otocyst 
is still functionless. This greater stability is explained when the 
Figure B. 
Lateral view of lobster larva of the third stage, showing swimming position. 
Magnified 6 diameters. 
swimming position of the body and appendages is observed (Fig. 5). 
The thoracic appendages are now relatively large, as compared with the 
size of the body. They are allowed to hang down ventrally, and in 
conjunction with the curved condition of the abdominal segments, serve 
to lower the position of the centre of gravity in the whole animal, thus 
rendering its swimming position much more stable. 
Figure C. 
Lateral view of lobster larva of fourth stage, illustrating the change in swimming position 
due to the presence of a functional otocyst. Magnified 6 diameters. 
Turning now to the fourth larval stage, we find the swimming posi- 
tion of the body entirely changed (Fig. C). The abdomen is no longer 
flexed and curved ventrally, but is held in approximately the same 
horizontal plane as the cephalothorax, while the thoracic appendages, 
instead of dragging downward through the water, are held up and for- 
ward in a line parallel with the long axis of the body. The great chelz 
project in front like the arms of a person preparing to dive, the exopods 
