288 PHYLUM ARTHROPOD A. 



opens by a bristle-guarded slit on the inner upper corner of 

 the expanded basal joint, and contains a gelatinous fluid 

 and small "otoliths," which appear to be foreign particles. 

 This " ear " seems to be an equilibrating organ, connected 

 with directing the animal's movements. In some other 

 Crustaceans the auditory hairs are lodged in an open de- 

 pression ; this has become an open sac in the crayfish, a 

 closed bag in the crab. Small setae- on the upper lip of 

 the mouth have been said to have a tasting function. 



The stalked eyes, which used to be regarded as append- 

 ages, arise in development from what are called " procephalic 

 lobes " on the head. They are compound eyes that is, 

 they consist of a multitude of elements, each of which is 

 structurally complete in itself. On the outside there is a 

 cuticular cornea, divided into square facets, one for each 

 of the optic elements ; beneath this lie, as in other parts 

 of the body, the nucleated epidermal cells. Then follows a 

 focussing layer, consisting of many crystalline cones. Each 

 crystalline cone is composed of four crystalline cells, which 

 taper internally, and externally secrete a firm crystalline 

 body. The bases of the crystalline cones are surrounded 

 by the retinula cells. Each retinula consists of five 

 elongated cells arranged about a central axis. Distally, 

 this axis is formed by the crystalline cone, proximally by 

 a little rod or rhabdom. The rhabdom consists of four 

 little red rods closely apposed together, and connected by a 

 nerve-fibre with the optic ganglion, which lies at the end of 

 the optic nerve. The proximal ends of the retinal cells are 

 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 

 retinule image is 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 

 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 



