238 ARTHROPODS. 



cone lies a retinula. The innermost part of this consists 

 of four little red rods, united closely into what is called a 

 rhabdome: Round about each rhabdome lie a number of 

 black retinula cells, and at the 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- 

 hne 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 it seems likely 

 that their eyes are able to form images of external objects. 



Alimentary System. — The food-canal consists of three dis- 

 tinct parts, a fore-gut or stomatodaeum developed by an in- 

 tucking 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 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 

 of course inadequate to supply sufficient carbonate of Ume 

 for the new skeleton, seem to have some relation to this 

 process. The occurrence of chitinous cuticle, hairs, teeth, 

 and gastroliths in the " stomach," is intelligible when the 

 origin of the fore-gut is remembered, and so is the dismantled 

 state of this region when moulting occurs. 



The mid-gut is very short, but it is the digestive and 

 absorptive region. From it, there grows out on each side a 

 large digestive gland with two ducts. This gland is much 



