328 PHYLUM ARTHROPODA 



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 rhabdom 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 con- 

 sists of internal rhabdom and external retinula cells. Be- 

 tween the individual optic elements lie some pigment cells. 

 The retinular 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 stomodasum developed by an 

 intucking from the anterior end of the embryo, a hind-gut 

 or proctodaeum 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 

 end of the body, so that the antennules and antennas lie far 

 in front of it. The fore-gut, which is lined by a chitinous 

 cuticle, includes a short " gullet," on the walls of which there 

 are small glands, hypothetically called " salivary," and a 

 capacious gizzard, which is distinctly divided into two 

 regions. 



In the anterior (cardiac) region there is a complex mill : in the 

 posterior (pyloric) region there is a sieve of numerous hairs. The mill 

 is very complex ; there are supporting " ossicles " on the walls with 



