THE CRAYFISH 139 



internal ends dipping down between the crystalline cones of 

 the ommatidea. In each ommatideum the crystalline cone is 

 an ovoid refractive body, surrounded by the four vitrellse 

 which have given rise to it. The cone itself is formed of four 

 pieces, one piece for each vitrella cell. The hinder ends of 

 the vitrellse abut upon the retinula. There are five elongated 

 retinular cells grouped together about the optical axis of the 

 ommatideum, and each gives rise along its inner margin, where 

 it is in contact with the remaining four retinular cells, to a 

 chitinous rod or rhabdomere. The five rhabdomeres are 

 united together to form the rhabdora. Each ommatideum is 

 surrounded by pigment cells. 



In the eye of Astacus, the cuticular lens surmounting each 

 ommatideum is followed by a group of four lentigen cells. 

 Beneath these is the " crystalline cone," a complex structure 

 formed by four crystalline cone cells or vitrellse, and consisting 

 of an outer refractive body, a median four-sided crystalline 

 cone proper and an inner refractive body {p.r.b., c.c, andi.r.b. 

 in fig. 32). The retinular cells are seven in number, and 

 co-operate in the formation of a spindle-shaped rhabdom, 

 which is shown in transverse section to consist of only four 

 rhabdomeres. The rhabdom is marked by transverse alter- 

 nately light and dark bands, and hence is often called the 

 striated spindle. The position and relation of the pigment 

 cells may be gathered from the figure. 



The general anatomy of the ovary and testis have already 

 been described. The latter resembles a racemose gland, 

 consisting of a number of ramifying ducts ending in small 

 dilatations. The cavities of the vesicles are fined by large 

 nucleated germinal cells, which undergo division, and their 

 products eventually become changed into remarkable wheel- 

 shaped spermatozoa. Each spermatozoon consists of a 

 discoid body produced at the circumference into a number 

 of tangential tapering curved processes. Inside the body is a 

 ring-shaped thickening, and lying to one side of this a smaller 

 oval corpuscle which is considered to be the nucleus. During 

 their passage through the vasa deferentia the spermatozoa are 

 enveloped in a viscid secretion, and on discharge are aggregated 

 into white' chalky-looking strings which are deposited by the 

 male on the sterna of the posterior thoracic and anterior 



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