CHAP. XIT. EVOLUTION IN CKUSTACEA. 123 



and abdomen at first, &c.) are neither to be deduced 

 from a retro-transfer of late-acquired advantages to this 

 early period of life, nor to be regarded at all as ad- 

 vantages over other Zoese which the larva might have 

 acquired in tbe struggle for existence. 



A similar development must have been once passed 

 through by the primitive ancestor of all Malacostraca, 

 probably differing from that of our Prawn, especially in 

 the circumstance that it would go on more uniformly 

 without the sudden change of form and mode of locomo- 

 tion produced in the latter by the simultaneous sprout- 

 ing forth and entering into action in the Nauplius of 

 four and in the Zoe'a of five pairs of limbs. It is to be 

 supposed that, not only originally but even still, in the 

 Iarva3 of the first Malacostraca, the new body-segments 

 and pairs of limbs are formed singly, first of all the 

 segments of the fore-body, then those of the abdomen, 

 and finally those of the middle-body, and, moreover, 

 that in each region of the body the anterior segments 

 were formed earlier than the posterior ones, and there- 

 fore last of all the hindermost segment of the middle- 

 body. Of this original mode more or less distinct traces 

 still remain, even in species in which, in other respects, 

 the course of development of their ancestors is already 

 nearly effaced. Thus the abdominal feet of the Prawn- 

 larva represented in fig. 33, are formed singly from 

 before backwards, and after these the last feet of the 

 middle-body ; thus, in Palinurus, the last two pairs of 

 feet of the middle-body are formed later than the rest ; 

 thus in the young larva? of the Stomapoda the last 



