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 (2) an outer layer of cells which 
secrete a long, lens-like body, the crystalline cone; (6) an 
inner layer of cells, called vefixuZe, which secrete in their 
common inner space the rabdomes, 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 
retinule 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 ¢actile hairs. 
These should be carefully distinguished, on the one hand, 
from mammalian hairs, and, on the other, from annelid sete. 
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 wsoph- 
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 gland, 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 
Alimentary. 
