PEENTISS : THE OTOCYST OF DECAPOD CRUSTACEA. 235 



thoracic appendages ; the abdominal segments are flexed ventrally, and 

 the thoracic eudopods, hanging down, steady the roUing motions some- 

 what. In the second stage the conditions are essentially the same. 



In the third stage the larvae 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. B). 

 The thoracic appendages are now relatively large, as compared with the 

 size of the body. They are allowed to hang down ventrally, and ia 

 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, illustratinpr 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 gi'eat chelae 

 project in front like the arms of a person preparing to dive, the exopods 



