556 ZOOLOGY 



SECT. 



Sensory Organs. — The eyes have the same essential structure 

 as the compound eye of Apus. The chitinous cuticle covering the 

 distal end of the eye-stalk is transparent, is divided by delicate 

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

 Beneath each facet of the cornea is an ommatidium, optically 

 separated from its neighbours by black pigment, and consisting 

 of an outer segment or vitreous body, and an inner segment or 

 retinula, 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 

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

 the antennulary nerve. The so-called auditory organ is a sac 

 formed by invagination of the dorsal surface of the proximal 

 segment, and is in free communication with the surrounding water 

 by a small aperture. The chitinous lining of the sac is produced 

 into delicate feathered auditory setm, 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. Probably the main function of these 

 organs is connected with the equilibration of the body rather than 

 with the sense of hearing. Many of the setae on the body 

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

 organs. 



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. 431, 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 testis (Fig. 441, B, t, u) lies in the thorax, just beneath the 

 floor of the pericardial sinus, and consists of paired anterior lobes 

 (t) and an upaired posterior lobe (w). From each side goes off a 

 convoluted vas deferens (vd), which opens on the proximal segment 

 of the last leg. The sperms are curious amoeboid bodies produced 

 into a number of stiff processes or pseudopodia (Fig. 23, /) : they 

 are aggregated into vermicelli-like spermatophores by a secretion 

 of the vas deferens. 



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

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

 oviduct (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 



