CHAP. XII. EVOLUTION IN CRUSTACEA. 129 



of Peneus and Palinurus, as also several peculiar larvae 

 of unknown origin, but which are in all probability to 

 be attributed to Macrurous Crustacea, necessitate on 

 the contrary the opposite supposition, namely, that the 

 different groups of the Macrura have passed from their 

 original to their present mode of development inde- 

 pendently of each other and also independently of the 

 Crabs." To this we may answer that the occurrence of 

 the Zoea-form in all the above-mentioned Decapoda, its 

 existence in Peneus during the whole of that period of 

 life which is richest in progress and in which the wide 

 gap between the Nauplius and the Decapod is filled up, 

 its recurrence even in the development of the Stoma- 

 poda, the occurrence .of a larval form closely approaching 

 the youngest Zoe'a of Peneus in the Schizopod genus 

 Euphausia, and the reminiscence of the structure of Zoe'a, 

 which even the adult Tanais has preserved in its mode 

 of respiration, all indicate Zoea as one of those steps 

 in development which persisted as a permanent form 

 throughout a long period of repose, perhaps through a 

 whole series of geological formations, and thus has also 

 made a deeper impression upon the development of its 

 descendants, and formed a firmer nucleus in the midst 

 of other and more readily effaced young states. It 

 cannot, therefore, surprise us that in transitions from 

 the original mode of metamorphosis to direct develop- 

 ment, even when produced independently, the larval 

 life commences in the same way with this Zoea-form in 

 different families, in which the earlier stages of de- 

 velopment are effaced. But except what is common to 



K. 



