83 . KEPORT — 1876. 



dimensions. In the Isopoda the sixth somite only has the coxte free, and the 

 appendages attached to them bear no very distant analogy to the homologous 

 pair as they exist in the Macrura. In numerous genera of Isopods the 

 sixth somite is developed to a very large size, and either absorbs or displaces 

 the terminal somite or telsou altogether, which in some genera is repre- 

 sented by a notch or cavity only, while in many others it is produced to a 

 point or terminates in a smooth and even margin ; with the exception of 

 some of the Anisopocl genera, the telson probably is absent throughout the 

 order of Isopods. 



The form of the pleon in the Brachyura bears as close a resemblance to 

 that of the Isopoda belonging to the tribe Liberatka as that of the Macrura 

 resembles Parasitica in the same order. 



The coxse or side-pieces, as they have been very commonly supposed to be, 

 are, in the Brachyura, very densely ossified with the dorsal arc, and this to 

 such an extent in the male animals that it is very difficult to determine their 

 presence. In the female, where the lateral development assumes a greater 

 extent, the line of union is capable of being determined by a marked depress 

 sion that defines the limit of the somites and the altered position of the appen.- 

 dao'es ; but that they are homologicallj' present in both sexes there can be no 

 reasonable cause of doubt. This, I think, may be generally depended on-^ 

 that the more the coxa departs from the normal typo of the joint, as we see 

 in the Macrurous Crustacea, and becomes associated with the dorsal arc of 

 the theoretical somite, the more the character of the appendage becomes sim- 

 plified or depreciated ; but, on the other hand, the more intimately it be- 

 comes associated with the ventral arc, the more it becomes developed in its 

 connexion with the requirements of the animal, and any variation of form is 

 dependant on the value of its position and the habits and necessities of the 

 creature. Thus we find that aU the appendages of the cephalon and pereion 

 are associated with the ventral arc in the Brachyura and Macrura, but in 

 the Edriophthalmia those of the pereion are associated with the dorsal arc ; 

 whereas the ap])eudages of the pleon are, in all divisions of Crustacea, so 

 intimately associated with the dorsal arc that in most cases the coxa is in- 

 corporated with the somite, and generally the remainder of the appendages 

 disappear or are reduced to merely a rudimentary condition, useful in some 

 females for the attachment of ova ; while in the males they disappear more 

 or less completely, or in the general conditions of Kfe become variated so as 

 to fulfil special requirements or peculiar functions. 



Thus the 21 somites of which the typical Crustacean consists each sup- 

 ports in its most simple condition a single pair of appendages ; and if we were 

 to suppose every segment of the animal to be reduced to its most simple cha- 

 racter, and the appendages attached to each segment reduced to the most 

 simple form of articulated hmbs, and all of them uniform in size, the animal 

 would bear a close analogy to a segmented annelid. 



This we must take as the archetype of a crustaceous animal, and assume 

 that the appendages are attached to the spaces that exist between the dorsal 

 and the ventral arcs of each somite. Thus when we observe any extreme 

 variation of form, we must consider the earliest and most simple condition of 

 the appendage in the archetype ; and it is not at variance with our idea of 

 progression to assume that any great departure from the most simple type 

 that appears to be common to the entire or a large portion of the subking- 

 dom of Crustacea had its origin at an earlier period in the history of its 

 evolution. 



The organs of vision arc common to all the Crustacea; and in those species 



