316 MR. WESTWOOD ON THE SUPPOSED EXISTENCE 



1. It is to be observed that the account given of the mode in which this meta- 

 morphosis is supposed to be effected, is as vague and indefinite as it is possible to be. 

 It is stated that the Zoe died in the act of casting its skin, but its metamorphosis 

 not being completed, prevented any knowledge being acquired of its general form ; 

 and yet it is added that five pairs of legs had become disengaged, and that the cha- 

 racters oi Zoea were entirely lost. Plate II. fig. 2., however, proves nothing like this. 

 The limbs of the future Crab are asserted to be beginning to show themselves, and yet 

 the Zoe retains its original form, without losing a single character which it previously 

 possessed, — without our being able to trace the least appearance of the animal having 

 commenced the shedding of the skin, — or without our being able to gain the least idea 

 how " the members, from being natatory and cleft (as shall shortly be shown), be- 

 come simple and adapted to crawling only." But Mr. Thompson has omitted to 

 fulfill this promise, which, if it mean anything, must be understood as an assertion 

 that the two pairs of natatory and cleft legs are transformed into five pairs of simple 

 crawling legs. 



2. The appearance of these limbs (represented as perfectly disengaged in Mr. 

 Thompson's Plate II. fig. 11.) previous to the shedding of the cephalothoracic shield 

 and anterior parts of the body, is totally at variance with the principles of ecdysis 

 observable throughout the Annulosa, in which the locomotive organs, at least the 

 legs, are the last which are disengaged, and the thoracic shield of the inclosed animal 

 the first portion exposed to view. It would, in fact, be impossible for the Zoe to 

 disengage the thoracic limbs without the thorax itself being previously withdrawn 

 from its covering. 



3. But we will look more precisely at the nature of this supposed disengagement 

 of the five pairs of legs. This, in the absence of any precise explanation given by 

 Mr. Thompson, may be presumed to be effected in three different ways. 



Firstly, — as indeed Mr. Thompson appears to suppose by his statement that the large 

 natatory limbs " become" simple ones, — this may be effected by the two pairs of large 

 natatory limbs entirely throwing away their outer covering, whereby the five pairs of 

 small simple legs, which had been previously inclosed within them, are disengaged. 

 This I take to be the true nature of the disengagement of the organs of motion in 

 the Annulosa ; but if we regard this to take place in Zoea, we shall necessarily have 

 two conditions totally at variance with the principles of ecdysis, viz. that an exist- 

 ing organ in a state of incomplete development incloses only a single organ, — thus, 

 the wings of the Grasshopper are not inclosed within the legs of its larva ; and that 

 an organ disengaged by the shedding of its envelope is always larger than such en- 

 velope. This, in fact, is the very end of the metamorphoses of the annulose animals, 

 the hardness of their outer covering preventing their growth, except by the shedding 

 of such covering. 



Secondly, We may imagine that the five pairs of minute rudimental legs of the 

 future Crab are not transformed from the two pairs of natatory limbs, but are totally 



