542 ZOOLOGY 



SECT. 



The nervous system (Fig. 448) consists, like that of Apus, 

 of a brain (g) and a ventral nerve-cord, united by cesophageal' 

 connectives (sc}. But the right and left halves of the ventral 

 cord have undergone partial fusion, so that the ganglia, and in 

 the abdomen the connectives also, appear single instead of double. 

 Moreover, the brain supplies not only the eyes and antennules, 

 but the antennae as well, and it is found by development that 

 the two pairs of ganglia belonging to the antennulary and antennary 

 segments have fused with the brain proper. Hence we have to 

 distinguish between a primary brain or archi-cerebrum, the ganglion 

 of the prostomium, and a secondary brain or syn-cerebrum formed 

 by the union of one or more pairs of ganglia of the ventral cord 

 with the archi-cerebrum. A further case of concrescence of ganglia 

 is seen in the ventral nerve-cord, where the ganglia of the last 

 three cephalic and first three thoracic segments have united to 

 form^ a large compound sub-cesophageal ganglion (bg). All the 

 remaining segments have their own ganglia, with the exception of 

 the telson, which is supplied from the ganglion of the preceding 

 segment. There is a visceral system of nerves (s) supplying the 

 gizzard, originating in part from the brain and in part from the 

 oesophageal connectives. 



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 have been 

 assigned the functions of smell and hearing respectively. The olfactory 

 organ is constituted by a number of extremely delicate olfactory 

 setcB, 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 setce, 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 (statocysts) 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 



