312 PHYLUM ARTHROPODA. 
segments, which exhibit marked division of labour; a 
comparison of Mebalia, Mysts, Euphausia, Penaeus, 
WVephrops, 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. Ita ice_ that in the 
Naupli i d in free-swimming copepods, 
ig antenna themselves aie—suumming Organs, Just as, 
however, in the Annelid head the locomo tion 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). Dhemelimentanmeneanal 
runs straight throughout the body; it consists of fore-gut, 
id-gut, and hind-gut. e-gut and hind are 
GER OaET POSLA Pagina ae ectoderm, and are 
always large, especially in ostraca. In the higher 
Malacostraca the fore-gut is furnished with a gastric mill: 
The mid-gut or archentéfon is always short, barns 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 
