312 PHYLUM ARTHROPODA. 



segments, which exhibit marked division of labour; a 

 comparison of Nebalia^ Mysis^ Euphausia, Penceus^ 

 NephropS) 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 append- 

 ages, like the parapodia of Annelids, are primitively organs 

 of locomotion, usually adapted as 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 in 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 append- 

 ages in connection with the mouth become modified in 

 connection with alimentation, and the further processes of 

 specialisation which differentiate the regions of the body 

 are reflected in the appendages of these regions. It is this 

 specialisation of certain appendages to function as mastica- 

 tory 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- 

 sented by a lobed ganglionic mass in the thorax, connected 

 with a mere rudiment, which corresponds to the abdominal 

 portion of the cord in the crayfish (Fig. 165). Sense 

 organs are usually well developed, and are not confined 

 to the head region ; thus many Mysids have " auditory " 

 organs in the tail (Fig. 164). The alimentary canal 

 runs straight throughout the body ; it consists of fore-gut, 

 mid-gut, and hind-gut. The fore-gut and hind-gut are 

 anterior and posterior invaginations of ectoderm, and are 

 always large, especially in Malacostraca. In the higher 

 Malacostraca the fore-gut is furnished with a gastric mill. 

 The mid-gut or archenteron is always short, but has con- 

 nected with it diverticula which form the so-called hepato- 

 pancreas. In the Entomostraca there is usually only a 

 single pair of outgrowths ; in Mysids, Cumacea. and larval 



