296 PHYLUM ARTHROPODA. 
in relation to its fresh-water habitat, where the risks of being swept 
away by currents are obviously great; but it must also be remem- 
bered that the tendency to abbreviate development is a general one. 
There is some maternal care in the crayfish, for the young are said 
sometimes to return to the mother after a short exploration on their 
own account. 
THE CRAB 
It is instructive to contrast the crab-type with that of the crayfish or 
lobster. The cephalothorax is broadened by a hollow extension of the 
gill-covering (branchiostegite) region. The abdomen is greatly re- 
duced, with a soft sternal region, and is bent permanently upwards 
and forwards in a groove in the thoracic sterna. In the male there are 
only two pairs of abdominal limbs, which have a reproductive function ; 
in the female there are four pairs, which carry the eggs. 
Fic. 149.—Section through cephalothorax of a crab, — 
After Pearson. 
#7., Heart; Te., extension of the tergum ; raeea sternum ; PL., pleuron; 
T., tendons; rst W.L., insertion of first walking leg; Bv., gill in gill- 
chamber; g., gut; d.a., descending artery; A., afferent branchial ; 
£., efferent branchial. 
The eye-stalks lie in sockets of the carapace; the bases of the 
reflexed antennules are also in sockets; the antennze are short and 
straight. 
The third maxillipedes are broad and flat and form a kind of oper- 
culum over the five preceding pairs of appendages. The great claws 
are relatively very large, the other thoracic legs are non-chelate, and 
in the swimming crabs, ¢.g. Portunus (see Fig. 150), the fifth pair of 
thoracic legs have their last joint adapted as a paddle. 
As to the soft parts, there is a noteworthy change in the nervous 
system. From the cerebral ganglia a pair of cesophageal commissures 
extend to a large ganglionated mass sheltered by the endosternal 
skeleton. It is composed of numerous pairs of ganglia fused together, 
and gives off nerves to maxillze, maxillipedes, and thoracic limbs. It 
is perforated by the sternal artery. The cesophageal commissures are 
united by a transverse commissure just behind the gullet, and in front 
of this cross junction there are two small ganglia giving off nerves to 
the mandibles. On the lower surface of the anterior part of the 
gizzard there are two small gastric ganglia innervated from the cerebrals. 
