282 PHYLUM AKTHROPODA. 



As usual, the nervous system arises from an cctodermic thickening. 

 The eye arises partly from the optic ganglia of the "brain,'' partly 

 from the "eye-folds," and partly from the epidermis. 



When the young crayfishes are hatched from the egg-shells, they still 

 cling to these, and thereby to the swimmerets of the mother. In most 

 respects they are miniature adults, but the cephalothorax is convex and 

 relatively large, the rostrum is bent down between the eyes, the tips of 

 the claws are incurved and serve for firm attachment, and there are other 

 slight differences. The noteworthy fact is that the development is com- 

 pleted within the egg-case, and that it is continuous without metamor- 

 phosis. The shortened life history of the crayfish is interesting 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 great extension of the 

 gill-covering (branchiostegite) region. The abdomen is greatly reduced, 

 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. 



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

 antennules are also in sockets ; both pairs of feelers are very short. 



The third maxillipedes are broad and flat and form a kind of operculum 

 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. Portnmts (see Fig. 133), 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 oesophageal 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 (see Fig. 134). 



When the branchial chamber is opened the large pyramidal gills are 

 seen, also the long sword-shaped epipodite (flabellum) of the first 

 maxillipede which seems to help the " baler," smaller also mobile 

 epipodites borne by the second and third maxillipedes, and the broad 

 scaphognathite of the second maxilla which bales the water forwards 

 and outwards. 



It must be clearly understood that the branchial chamber is entirely 



