ALIMENTARY SYSTEM. 261 



cones. Each crystalline cone is composed of four crystalline 

 cells, which taper internally. Internal to each crystalline 

 cone lie a number of retinula cells. The innermost of 

 these surround four little red rods, united closely into what 

 is called a rhabdome. At its base, a nerve fibre enters 

 from the adjacent optic ganglion at the end of the optic 

 nerve. Thus each element consists of corneal facet, crystal- 

 line cone, and retinula, and the retinula consists of internal 

 rhabdome, and external retinula cells. Between the in- 

 dividual optic elements, lie some pigment cells. Opinions 

 differ as to the visual powers of Crustaceans, but their eyes 

 are able to form images of external objects, and these 

 images are erect, not inverted as in the eyes of Vertebrates. 



Alimentary System. 



The food canal consists of three distinct parts, a fore 

 gut or stomatodaeum developed by an intucking from the 

 anterior end of the embryo, a hind gut or proctodasum 

 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 antennae 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, or " stomach," 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, but 

 there is no difficulty in dissecting it carefully, nor in seeing 

 at once that there are supporting " ossicles " on the walls 

 with external muscles attached to them, and internally 

 projecting teeth which clash together and grind the food. 

 Three of the teeth are conspicuous ; a median dorsal tooth 

 is brought into contact with two large laterals. On each 

 side of the anterior part of the gizzard, there are two limy 

 discs or gastroliths, which are broken up before moulting, 

 and though quite inadequate to supply sufficient carbonate 

 of lime for the new skeleton, seem to have some relation to 

 this process. The occurrence of chitinous cuticle, hairs, 



