196 ARTHROPODA [CH. 



ova. The nucleus is in the centre at the bottom of a chitinous tube 

 which seems capable of being turned inside out and thus forcing the 

 nucleus into the ovum. The male duct or vas deferens is long and 

 coiled, and for part of its length is lined by cells which produce a 

 milky fluid in which the male cells float. When this fluid with its 

 contained cells escapes from the male pore it is received by the tube 

 formed by the two first pairs of appendages of the abdomen, and the 

 male then seeks the female and turning her on her back, discharges 

 the fluid on to the eggs. 



Nothing could more strongly illustrate the proclivity which the 

 protoplasm of the Arthropoda has towards the production of cuticle 

 than the substitution of these motionless male cells with their spiny 

 covering for the active motile spermatozoa. One cannot but believe 

 that spermatozoa would be more advantageous, especially for aquatic 

 animals, and other groups of the Arthropoda such as the Arachnida, 

 have managed to retain them, but in these Crustacea the set towards 

 the production of cuticle appears to be so strong that they have 

 become an impossibility. The study of the development of Peripatus 

 teaches us, that not only are the genital organs of Arthropoda 

 remnants of coelomic sacs but that the genital ducts are the remnants 

 of the coelomiducts or " nephridia " opening into these sacs. There- 

 fore, in the crayfish there are three pairs of " nephridia " left ; one 

 pair acting as kidneys, one as oviducts, one as vasa deferentia. 



The nervous system of the crayfish is constructed on the same 

 type as that of the Annelida, differing in details only, though these 

 details are most instructive. It consists of a brain, situated in front 

 of the mouth, communicating by means of a nerve-collar, which 

 encircles the oesophagus, with a ventral chain of ganglia. The 

 differences are as follows. In the Annelida the nuclei of the 

 neurons are scattered all along the ventral cord, being only less 

 numerous in the commissures than in the ganglia. In the Crus- 

 tacea, and indeed all Arthropoda except Peripatus, the nuclei are 

 confined to the ganglia and the commissures consist of axons (nerve- 

 fibres) only. Further, in most Annelida the cord is apparently a 

 single one, though the microscope shows that it consists of a 

 double strand of fibres but in the crayfish it is clearly a double 

 one in thorax and apparently a single one in the abdomen. 



Finally the ganglia of several adjacent segments show in some 

 cases a tendency to coalesce so that there are considerably fewer 

 ganglia than segments as indicated by the appendages. Thus the 

 brain supplies nerves not only to the eye region which we may 

 suppose to represent the prostomium of Annelida but also to the 



