124 HISTOEY OF CRUSTACEA. Chap. XII. 



three abdominal segments are destitute of limbs, wbich 

 are still wanting on the last of them in older larvae ; 

 and thus, in the Isopoda, the historically newest pair of 

 feet is produced later than all the rest. In the Cope- 

 poda this formation of new segments and limbs, gradu- 

 ally advancing from before backwards, is more perfectly 

 preserved, than in any of the higher Crustacea.^ 



The original development of the Malacostraca start- 

 ing from the Nauplius, or the lowest free-living grade 

 with which we are acquainted in the class of Crustacea, 

 is now-a-days nearly effaced in the majority of them. 

 That this extinction has actually taken place in the way 

 already deduced as a direct consequence from Darwin's 

 theory, will be the more easily demonstrated, the more 

 this process is still included in the course of life, and 

 the less completely it is already worn out. We may 

 hope to obtain the most striking examples in the still 

 unknown developmental history of the various Schizo- 

 poda, Peneidae, and, indeed, of the Macrura in general. 

 At present the multifarious Zoea-forms appear to be 



1 It is well known that, in many cases, even in adult animals the last 

 segment of the middle-body, or some of its last segments, either want 

 their limbs or are themselves deficient {Entoniscus Porcellanse S, Leucifer, 

 &c,). This might be due to the animals having separated from the 

 common stem before these limbs were formed at all. But in those 

 cases with which I am best acquainted, it seems to me more probable 

 that the limbs have been subsequently lost again. That these particular 

 limbs and segments are more easily lost than others is explained by the 

 circumstance that, as the youngest, they have been less firmly fixed by 

 long-continued inheritance. ("Mr. Dana believes, that in ordinary 

 Crustaceans, the abortion of the segments with tlieir appendages almost 

 always takes place at the posterior end of the cephalothorax." — Darwin, 

 BalanidsB, p. 111.) 



