274 



PHYLUM ARTHROPODA. 



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 

 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 stomoda^um developed by an 

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



FIG. 127. Longitudinal section of lobster, showing some 



of the organs. 



//., Heart; AO.< ophthalmic artery; aa., ancennary artery; ah., 

 hepatic artery; ST., sternal artery; ^>'^., 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 ; 7"., testes. 



or proctoda^um 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, hypothetical!} 7 called " salivary," and a 

 capacious gizzard, which is distinctly divided into two 

 regions. 



