ON OUR PRESENT KNOWLEDGE 01' THE CRUSTACEA. 15 



many instances of variation in form, not only between these two pairs, but 

 between the appendages of the same pair. This variation is generally estimated 

 as of sufficient importance for generic classification ; but whatever variation 

 may exist, it is one of degree only, and not inconsistent with the same 

 functional characters. These are so universally connected with cither seizing 

 or directing food to the mouth, that I do not think that a better term can 

 be suggested than gnathopoda for tho eighth and ninth pairs of appendages, 

 which arc the first two that belong to the pereion. 



The five following pairs of appendages (that is, the true pereiopoda or 

 perambulatory legs) correlate between organs adapted for walking, grasping, 

 and swimming. When adapted for walking, we consider them in their 

 normal condition ; they consist then of seven simple subcylindrical joints, 

 of which the last is formed into a simple pointed extremity. 



When intended for grasping or holding any object, the anterior distal 

 angle of either joint of the leg may be produced to a corresponding process 

 against which the extremity of the ultimate joint impinges, and so forms a 

 prehensile organ that represents a two-fingered hand. 



The power of producing a chelate process appears to exist in the various 

 joints of all the five pairs of pereiopoda. 



This, I think, is strongly exemplified in the presence of chelae, more or 

 less perfect, in the young of many Crustacea, which disappear in the adult 

 stage. Thus in the freshwater Astacus, while in the embryonic condition 

 rudimentary chelae are apparent in each pair of appendages, they are found fully 

 developed only on the three anterior pairs of the adult. Again, wo frequently 

 see, both in this genus as well as in Cancer, that supplementary chelae are 

 developed at various parts of these appendages, not only when they are not 

 wanted, but often when they are absolutely detrimental to the animal's 

 requirements. 



In some forms we find the chela is prominently developed in the larval or 

 young stage, while it entirely disappears in the adult. Evidence of this exists 

 in some genera of the Hyperine Amphipoda, such as Vibilia and Brachysceltts. 

 In the adult Vibilia the leg is long, slender, and simple, not very unlike 

 that of the same appendage in the normal character of Amphipoda; in 

 Bracliyscelus it has the basis developed to a large scale, and the remaining 

 five joints are reduced to little more than a rudimentary limb. In these very 

 dissimilar genera the penultimate pairs of pereiopoda are in the larval condition 

 developed into chelate appendages ; in the former genus by a process attached 

 to the carpus, in the latter by a similar process attached to the propodos. 



These animals probably resemble Hyperia in their habits, and pass most of 

 their lives within the cavity of some Medusa-like creature, where 

 prehensile appendages are of little use, and consequently the force of 

 chelate production is not stimulated. It is an interesting problem, and pro- 

 bably true, that the not very remote ancestor of cither or both these genera 

 is to be found in a form not very distant from one like Phronima, where the 

 chelate organ is large and well developed in the antepenultimate pair of pereio- 

 poda of the adult animal. 



In some genera, as Gclasimtts, the difference in extent of development is 

 very great between the right chela and the left, and between those of the 

 males and females. When, in the latter case, the part varies from the type, 

 the variation generally exists in the male animal, the chela) of the female, 

 and less altered appendage in the male, corresponding generally with the 

 normal form. In Oelasimus the variation in the male is so great that the 

 chief characteristic limb, which may be either of the first pair of pcreio- 



