296 PHYLUM ARTHROPODS. 



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. 



FIG. 149. Section througli cephalothorax of a crab. 

 After Pearson. 



The eye-stalks lie in sockets of the carapace ; the bases of the 

 reflexed antennules are also in sockets; the antennae 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, e.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 maxillae, 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. 



