256 



CRUSTACEA. 



deeply pigmented. Thus each element consists of corneal 

 facet, crystalline cone, and retinula, and the retinula consists 

 of internal rhabdome, and external retinula cells. Between 

 the individual optic elements lie some pigment cells. The 

 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 stomodseum developed by an 

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



Fig. 107. — Longitudinal section of lobster, showing some 

 of the organs. 



H., Heart ; AO., ophthalmic artery ; aa., antennary artery; ah., . 

 hepatic artery ; ST., sternal artery; SA., superior abdominal 

 artery; MG., mid-gut ; DG. , digestive gland ; HG., hind-gut; 

 Ex., extensor muscles of the tail ; FL, flexor muscles of the tail ; 

 I A., inferior abdominal artery ; G., gizzard; C, cerebral ganglia; 

 P., pericardium ; T., testes. 



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 antennae lie far 

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

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

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

 capacious gizzard, which is distinctly divided into two 

 regions. 



