514 ZOOLOGY SECT. 



lines into square areas or facets, and constitutes the cornea. Be- 

 neath each facet of the cornea is an ommatideum, optically 

 separated from its neighbours by black pigment, and consisting 

 of an outer segment or vitreous /"///, and an inner segment or 

 ri'fiiti'lit formed of sensory cells enclosing a rhabdome. 



The antennules contain two sensory organs, to which are assigned 

 the functions of smell and hearing respectively. The olfactory 

 organ is constituted by a number of extremely delicate olfactory 

 setae, borne on the external flagellum, and supplied by branches of 

 the antennulary nerve. The an<H1<>ri/ i>r</mt is a sac formed by 

 imagination of the dorsal surface of the proximal segment, and is 

 in free communication Avith the surrounding water by a small 

 aperture. The chitinous lining of the sac is produced into delicate 

 feathered otn/tftir// sctcr, supplied by branches of the antennulary 

 nerve, and in the water which fills the sac are minute sand- 

 grains, which take the place of otoliths, but, instead of being 

 formed by the animal itself, are taken in after each ecdysis, 

 when the lining of the sac is shed. Many of the seta 2 on the 

 body generally have a definite nerve-supply, and are probably 

 tin-tile organ*. 



Reproduction. The Crayfish is dioecious, and presents a very 

 obvious sexual dimorphism. The abdomen of the female is much 

 broader than that of the male: the first and second pleopods of 

 the male are modified into tubular or rather spout-like copulatory 

 organs (Fig. 297, 9} ; and the reproductive aperture is situated in 

 the male on the proximal podomere of the fifth leg, in the female 

 on that of the third. 



The tcstis (Fig. 407, B, t, u} lies in the thorax, just beneath the 

 floor of the pericardia! sinus, and consists of paired anterior lobes 

 (t) and an unpaired posterior lobe (n). From each side goes off a 

 convoluted r.s defer ens (vd), which opens 011 the proximal segment 

 of the last leg. The sperms are curious non-motile bodies pro- 

 duced into a number of stiff processes (Fig. 20, /): they are 

 aggregated into vermicelli-like spermdtophores by a secretion of the 

 vas deferens. 



The ovary (A, or. u) is also a three-lobed body, and is similarly 

 situated to the testis : from each side proceeds a thin- walled 

 drill in-t. (od); which passes downwards, without convolutions, to 

 open on the proximal segment of the third or antepenultimate 

 leg. The eggs are of considerable size and are centrolecithal. 



As in Apus, both ovary and testis are hollow organs, discharging 

 their products internally. The ova, when laid, are fastened to the 

 set;e on the pleopods of the female by the sticky secretion of 

 glands occurring both on those appendages and on the segments 

 themselves: they are fertilised immediately after laying, the male 

 depositing spermatophores on the ventral surface of the female's 

 body just before oviposition. 



