338 



PHYLUM ARTHROPODA 



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

 refiexed 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. i8i), the fifth pair of 

 thoracic legs have their last joint adapted as a paddle. 



There is a noteworthy change in the nervous system (see Fig. i8i). 

 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 maxilla?, maxillipedes, and thoracic limbs. It is perforated 

 by the sternal artery. The oesophageal commissures are united by a 



Fig. 183. — Ventral aspect of female shore crab. 

 Abd., Abdomen ; mxp., third maxillipede. 



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. 



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," the smaller and 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 

 outside of the body, being formed by the lateral extension of a hollow 

 reduplicature from the tergal region. 



The large gizzard, the enormous greyish-yellow hepatopancreas, the 

 transparent pericardium, and other organs are readily seen. 



