170 FUNCTION OF THE SPINES OF THE CRUSTACEAN ZOCEA. 



progress will be in a succession of ill-directed spirals^ and it will be 

 accompanied by tliat peculiar rotation about the long axis of the body 

 which is seen in larv83 whose locomotion is effected by means of 

 ciliated girdles. And not only will all forward movement be thus 

 indirect, but it will be impeded by frequent and apparently involun- 

 tary somersaults, after which each Zooea will hang for a moment 

 vertically in the water, as if to recover its sense of direction. 



It is hard to avoid the supposition that this power of rapid and 

 direct motion which accompanies the possession of long spines must 

 be an advantage to its possessor ; but if this be the case, it is difficult 

 to imagine the causes which have led to the abolition of such spines 

 during the passage from the Peneus group to such forms as Crangon, 

 Palcemon, &c., which are numerous and widely spread, while their 

 larvEe are devoid of spines and incapable of executing the rapid and 

 skilful movements in question. 



