GENERAL NOTES ON CRUSTACEANS. 297 



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 

 Neb ali a ) 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- 

 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. 143). Sense 

 organs are usually well developed, and are not confined 

 to the head region ; thus many Schizopods have " auditory ' : 

 organs in the tail (Fig. 142). 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 Malacostraca 

 the fore-gut is furnished with a gastric mill. The mid-gut 

 or archenteron is always short, but has connected with it 

 diverticula which form the so-called hepato-pancreas. In 

 the Entomostraca there is usually only a single pair of out- 

 growths ; in Schizopods, Cumacea, and larval Decapods there 

 are three pairs ; a process of rapid growth and branching 

 converts these into the compact digestive gland of the adult 

 Decapods. In connection with the posterior end of the 

 mid-gut in Amphipods and some others, there is a pair of 

 blind tubes functioning as excretory organs, and presenting 

 an interesting similarity to the Malpighian tubes of insects, 

 which, however, are in connection with the hind-gut. The 

 body cavity is never large, being mainly filled up with 



