NEPHROPS, 215 



complete optical apparatus. The ommatidia are arranged 

 radially, converging to the centre of the eye towards the 

 optic ganglion, and their outer ends are covered by a 

 thickened cuticle divided into facets. Each ommatidium 

 or eye-element consists of (a) an outer layer of cells which 

 secrete a long, lens-like body, the crystalline cone; (b) an 

 inner layer of cells, called retinulce, which secrete in their 

 common inner space the rhabdomes, or rod-like bodies. 

 From these there pass fine nerves to the optic ganglion, 

 which in its turn communicates with the brain. The 

 crystalline cones form the dioptic apparatus, and the 

 retinulse and rhabdomes are the sensory apparatus. Between 

 the ommatidia, cells loaded with pigment grow up from the 

 connective-tissue layers below. They serve to isolate the 

 ommatidia and shut out cross-rays. 



(2) The otocysts consist of paired hollow cavities in the 

 base of the antennule. Each communicates with the ex- 

 terior by a minute aperture. The cavity contains a few 

 sand-grains, and its wall has sensitive ''hairs" projecting 

 into the cavity, supplied by fibres from the antennulary 

 nerve. (3) A number of the "hairs" on the antennule 

 are sensory and are said to have an olfactory sense. (4) 

 Crustacea^ with a hard exoskeleton, can hardly have the 

 tactile sense distributed all over the surface like some other 

 animals, but they have numerous sensory or tactile hairs. 

 These should be carefully distinguished, on the one hand, 

 from mammalian hairs, and, on the other, from annelid setae. 

 The seta is a cuticular bristle formed of chitin throughout, 

 but the lobster's "hair" consists of a delicate cuticle on the 

 surface and a living protoplasmic axis connected by sensory 

 nerve to the nerve cord. 



The mouth, as we have seen, passes from the antero- 

 ventral mid-line past the mandibles through a short oesoph- 

 agus into the spacious stomach. This is divided 

 by a constriction into two parts, the so-called 

 cardiac and pyloric chambers. The pyloric chamber leads 

 into a short mesenteron, into which open the paired ducts 

 from a large digestive gla?id^ and thence into an intestine to 

 the anus on the ventral surface of the telson. 



Development teaches us that the whole of the alimentary 

 canal, except the mesenteron, arises from ectoderm, and, in 



