288 THE CRUSTACEA 



This is the case in the Brachyura and the majority of the Anomura. 

 In the few Natantia, however, which have closed statocysts, solitary 

 statoliths, probably of organic composition, are present, which are 

 no doubt formed in situ. As with the open statocysts, the lining 

 membrane, and with it the statolith, is cast and renewed at each 

 ecdysis. In all cases the inner surface of the statocyst bears 

 plumose sensory setae arranged in one or more rows. In Leucifer 

 the tips of the sensory hairs are embedded in the substance of the 

 statolith. 



The development of the statocysts has been traced in the 

 Lobster and the Shore-crab. In both cases the functional state is 

 assumed rather suddenly ; at the fourth larval stage in the lobster 

 and the first Megalopa-stage in the Crab. In the latter the statocyst 

 is at first open to the exterior and sand-grains are found in it. 



Sensory filaments occur in most cases on the external flagellum 

 of the antennules, commonly in larger numbers in the male than in 

 the female sex. In many Caridea they are confined to a specially 

 thickened portion of the flagellum, and when the flagellum 

 bifurcates the filaments are borne by the secondary branch 

 (Fig. 156, B, p. 265). In some terrestrial species (CoenoUta) the 

 filaments are very short, forming a close fur. 



Phosphorescent organs are now known in a number of deep-sea 

 Decapoda (Sergestidae, Penaeidae, Hoplophoridae, Pandalidae, 

 Eryonidae), but the nature of the organs differs widely in the 

 different groups. In Aristeus coruscans (Penaeidae) (Fig 168) 

 and Heterocarpus alphonsi (Pandalidae) (Fig. 148, p. 259) Alcock 

 observed a luminous fluid to be emitted from the base of the 

 antennae, apparently from the orifices of the antennal glands. 

 This case may be compared with that of Gnathophausia among 

 the Mysidacea, where a luminous secretion is produced by a gland 

 on the maxilla which may possibly be the excretory maxillary gland. 

 Polycheles phosphorus (Eryonidae) was found by the same observer 

 to be " luminous at two points between the last pair of thoracic 

 legs, where there is a triangular glandular patch." Numerous 

 phosphorescent organs have been found by Coutiere on the body 

 and limbs of various Hoplophoridae, but their structure has not 

 been examined. In Sergestes challengeri Hansen has found an 

 extraordinary number of luminous organs (about 150) on the 

 body and limbs, although they are not found in other species of 

 the genus. In this case the structure recalls that found in the 

 Euphausiacea. Each organ has, internally, a reflector, composed 

 apparently of concentric lamellae, enclosing a mass of cells. 

 There is nothing corresponding to the "striated body" of 

 Euphausiacea, and the "lens" is double, the outer part being 

 formed by a thickening of the cuticle which has no counterpart in 

 the Euphausiacea. 



