THE CRUSTACEA 19 



divisible into rhabdomcrcs corresponding in number to the retinula 

 cells by which it is formed. At their bases the retinula cells pass 

 into nerve-fibres (/) which penetrate the basement membrane (b) and 

 , enter the optic ganglia. Each ommatidium is more or less com- 

 pletely isolated by a sheath of black pigment contained partly in 

 the retinular cells, partly in special cells lying between them. By 

 movements in the protoplasm of these cells the position of the 

 pigment changes according to -the amount of light falling upon the 

 eye. A layer of reflecting pigment, the tapetum, may also be 

 present. It is impossible here to summarise the details of histo- 

 logical structure or of physiological interpretation. It may be 

 stated, however, that the variations in structure found in different 

 Crustacea appear to be determined not so much by the systematic 

 affinities as by the habits of the organisms. In this connection 

 the work of Chun and of Doflein on the structure of the eyes in 

 pelagic and abyssal species may be referred to. 



In many Crustacea, as already stated, the paired eyes are set 

 on movable peduncles, and it is probable that this condition is the 

 primitive one. In the primitive Branchiopoda the eyes are either 

 pedunculated or, if sessile, movable in such a way as to suggest 

 derivation from the pedunculate condition. 



Other Sense-Organs. The other sense-organs of the Crustacea^ 

 are formed by modification of the hairs or setae on the surface of 

 the body and limbs (Fig. 14). As in other Arthropoda, many of 

 these setae are tactile. Each consists of a hollow conical outgrowth 

 of the cuticle, movably articulated at the base and containing 

 a prolongation of some of the cells of the hypodermis. One or 

 more nerve-fibrils may be traced into the interior, and, in some 

 cases, a ganglion-cell in connection therewith lies at the base of 

 the seta. When feathered, or provided with secondary barbs, the 

 setae will respond to movements or vibrations in the surrounding 

 water, and to some setae of this type an auditory function has 

 been attributed (Fig. 14, C). In certain Malacostraca more 

 specialised organs are found which have been regarded as auditory. 

 In most Decapoda there is an invagination of the integument in 

 the basal segment of the antennule having plumose " auditory " 

 setae on its inner surface. In some cases the sac remains 

 open to the exterior, permitting the introduction of sand -grains 

 which act as " otoliths." In other cases the sac is completely 

 closed, and may then contain a single " otolith " secreted by its 

 Avails. Otocysts are found in a few other Malacostraca in various 

 positions ; for example, in the endopodites of the uropods in the 

 Mysidae. 



Eecent investigations have rendered it doubtful, however, 

 whether aquatic Crustacea can hear at all, in the proper sense of 

 the term, and it has been shown that one function, at least, of the 



