288 PHYLUM ARTHROPODA, 
opens by a bristle-guarded slit on the inner upper corner of 
the expanded basal joint, and contains a gelatinous fluid 
and small “ otoliths,” which appear to be foreign particles. 
This “ear” seems to be an equilibrating organ, connected 
with directing the animal’s movements. In some other 
Crustaceans the auditory hairs are lodged in an open de- 
pression; this has become an open sac in the crayfish, a 
closed bag in the crab. Small sete on the upper lip of 
the mouth have been said to have a tasting function. 
The stalked eyes, which used to be regarded as append- 
ages, arise in development from what are called “ procephalic 
lobes” on the head. They are compound eyes—that is, 
they consist of a multitude of elements, each of which is 
structurally complete in itself. On the outside there is a 
cuticular cornea, divided into square facets, one for each 
of the optic elements; beneath this lie, as in other parts 
of the body, the nucleated epidermal cells. Then follows a 
focussing layer, consisting of many crystalline cones. Each 
crystalline cone is composed of four crystalline cells, which 
taper internally, and externally secrete a firm crystalline 
body. The bases of the crystalline cones are surrounded 
by the retinula cells. Each retinula consists of five 
elongated cells arranged about a central axis. Distally, 
this axis is formed by the crystalline cone, proximally by 
a little rod or rhabdom. The rhibdom consists of four 
little red rods closely apposed together, and connected by a 
nerve-fibre with the optic ganglion, which lies at the end of 
the optic nerve. The proximal ends of the retinal cells are 
deeply pigmented. Thus each element consists of corneal 
facet, crystalline cone, and retinula, and the retinula consists 
of internal rhabdom and external retinula cells. Between 
the individual optic elements lie some pigment cells. The 
retinule image is erect, not inverted as in the eyes of 
Vertebrates. 
Alimentary system.—The food canal consists of three 
distinct parts—a fore-gut or stomodaum developed by an 
intucking from the anterior end of the embryo, a hind-gut 
or proctodzeum similarly invaginated from the posterior end, 
and a mid-gut or mesenteron, which represents the original 
cavity of the gastrula. 
The mouth has been shunted backwards from the anterior 
