THE DECAPODA 287 



tion is somewhat less complete, the outlines of five pairs of ganglia 

 can be distinguished in the central mass, while posteriorly is a 

 shortened chain of five pairs of ganglia corresponding to the 

 abdominal somites, although not extending beyond the posterior 

 limits of the thorax. 



A system of visceral nerves is well developed in the Decapoda. 

 A gastric plexus is formed by anastomosis of three nerves, a median 

 one arising from the posterior surface of the cerebral ganglia and 

 a pair from the oesophageal commissures. Special nerves to the 

 rectum are given off by the last abdominal ganglion. 



Sense-Organs. The paired eyes are well developed in the great 

 majority of Decapods, although, as already mentioned, they may 

 be reduced or entirely absent in deep-sea and cave-dwelling forms 

 as well as in some parasitic and burrowing species. The cornea is 

 generally distinctly faceted, the facets being square or hexagonal 

 in outline, Sometimes they are square in the centre of the corneal 

 area and hexagonal towards the margin. As a rule the crystalline 

 cone is formed by four cells, and there are seven retinular cells 

 enclosing a quadripartite rhabdome. The nauplius-eye has been 

 found to persist in a vestigial condition in the adult in many of 

 the lower Decapoda. 



A pair of statocysts lodged in the proximal segment of the anten- 

 nules occur in the great majority of Decapods (Fig. 156, A, st, p. 265). 

 Only in certain Caridea do these organs appear to be entirely 

 wanting (Pandalus, Hippolyte). The statocyst develops as an 

 invagination of the integument, and in most of the lower Decapods 

 it remains in communication with the exterior, sometimes by a 

 wide aperture (Crangon), more commonly by a narrow slit. Rarely 

 among the Natantia the statocyst appears to be quite closed 

 (Leucifer, Sergestes), and this is the case also in the Galatheidea 

 and Hippidea among the Anomura and in the whole of the 

 Brachyura. In the Brachyura, after ecdysis, the statocyst is open 

 to the exterior by a narrow slit, which, however, soon closes by 

 coalescence of the newly formed cuticle covering its edges. In this 

 group also the cavity assumes a complex form by the folding of 

 its walls. In those cases where the statocyst remains open it 

 contains a number of foreign particles, sand-grains, which act as 

 statoliths, and are in some cases agglutinated together into a mass 

 by an organic substance secreted by dermal glands on the inner 

 surface of the sac. In this mass the tips of the sensory setae 

 are embedded. When ecdysis takes place the chitinous cuticle 

 lining the statocyst is thrown off and with it the contained sand- 

 grains, and it has been shown that fresh grains are introduced by 

 the animal either burying its head in the sand or placing the 

 grains in position by means of its chelae. When the statocyst is 

 without external opening it usually contains no solid particles. 



