ALIMENTARY CANAL OF ARTHROPODA. 267 



marks off each separate area (D). (Faceted eye of the higher 

 Crustacea and of the Insecta.) 



The number of the elements which make up one of these eyes, as 

 well as their size, and the form taken by the several parts, may 

 undergo various modifications. The crystalline cones are generally 

 present in this kind of eye in the Crustacea, but in many Insecta 

 the cells (crystalline cells), which in other eyes are differentiated 

 into crystalline cones, persist without forming crystalline cones. 

 Lastly, in many, the pi'imitive condition of the retinula, in which the 

 separate cells, each with its rod, may still be made out, persists 

 (Tipulida?). The projecting character of the eye, owing to its curva- 

 ture, may lead to a stage in which the eye is stalked. When still 

 more developed this stalk may be movable (Podophthalmata). 



Grexacher, H., Untersnchnugeu iiber das Arthropodenange. Beilagehcft zu 

 den klin. Monatsbl. fur Augenheilkunde. XV. .Tahrgang. 



Alimentary Canal. 



§ 207. 



The enteric canal of the Arthropoda is differentiated in much the 

 same way as in Vermes. The endoderm encloses what yolk-material 

 is not used up in the early stages of differentiation, and this is 

 absorbed during the gradual processes of further development. The 

 mouth and anus, and the connected parts of the enteron, are not 

 developed according to any general rule. When the enteric walls 

 are completely differentiated the nutrient canal forms a tube, which 

 traverses the whole length of the ccelom, and is but seldom adapted 

 to the metameres of the body ; it begins by a mouth, which is placed 

 on the ventral surface, and extends to an anus, which, as a rule, is 

 placed in the last metamere. The external chitinous covering of the 

 body extends into the fore- and hind-gut; in the mid-gut, which is 

 formed from the endoderm, it is replaced by a soft cuticle. The 

 appendages (§ 189), converted into masticatory and other organs, are 

 grouped round the mouth, and a process, which is formed from the 

 integument, joins them as an upper lip. 



§ 208. 



The enteric canal of the Crustacea is distinguished by the 

 straightness of its course, and the slight extent to which its divisions 

 are complicated. The mouth, which is ventral, is often placed some 

 way back, so that the fore-gut, which arises from it, runs forward 

 at lirst, and then turns backwards at a sharp angle. The terminal 

 portion of the ordinarily narrow fore-gut is known as the pharynx 

 or oesophagus ; it is generally widened and distinctly mai-ked off 

 from the succeeding mid-gut ; in many forms a wedge-like process 



