GENERAL NOTES ON CRUSTACEANS. 275 



notice the two pairs of antennas, the presence of carbonate 

 of lime in the cuticle, and the nature of the respiratory 

 organs — these, with few exceptions, being adapted for breath- 

 ing in water. While these characters remain constant 

 throughout the group, there is an almost infinite variation 

 in detail. In regard to the segmentation of the body, we 

 notice that, apart from the general tendency to reduction 

 which is so marked in many parasitic forms, the higher 

 forms as compared with the lower show marked specialisa- 

 tion. In the primitive Phyllopods the body consists of a 

 large but varying number of segments, remarkably uniform 

 in structure. The higher Crustacea, on the other hand, are 

 characterised by their relatively few but constant segments, 

 which exhibit marked division of labour ; a comparison of 

 Nebalia, Schizopods and Decapods, a series which illustrates 

 the development of the thorax, will make this plain. The 

 same gradual process of specialisation is observable in the 

 appendages. Typically consisting of a basal piece and two 

 branches, the appendages, like the parapodia of Annelids, 

 are primitively organs of locomotion; in the Crustacea 

 especially, swimming organs. In Phyllopods the great 

 majority of the appendages remain permanently at this 

 level. It is worth notice that in the Nauplius and in 

 Ostracods and the free-swimming Copepods, the antennae 

 themselves are swimming organs. Just as, however, in the 

 Annelid head the locomotor function of the parapodia 

 becomes subordinated to the sensory one, so also in 

 Crustacea the anterior appendages of the head become 

 specialised as sense organs. Again, the appendages in 

 connection with the mouth become modified in connection 

 with alimentation, and the further processes of specialisa- 

 tion which differentiate the regions of the body are reflected 

 in the appendages of these regions. A comparison of 

 Nebalia, Schizopods and Decapods, will again make this 

 plain. It is this specialisation of certain appendages to 

 function as masticatory organs which especially characterises 

 Arthropods as compared with Annelids. 



In the nervous system there is always a certain amount 

 of fusion of ganglia- — these never being so numerous as the 

 segments — but the fusion is more marked in the more 

 specialised forms. In the Crabs the ventral chain is repre- 



