xxvn ABSENCE OF CILIA 319 



The Crayfish is dioecious. The ovaries (ovy) are a pair 

 of hollow organs, united in the middle line in some genera, 

 situated in the thorax, and opening by oviducts (B, ovd] on 

 the bases of the third pair of legs. The spermaries (testes) 

 are also frequently united in the middle line and open 

 by spermiducts (vasa deferential on the bases of the fifth 

 pair of legs. There is some reason for thinking that the 

 gonaducts represent modified nephridia, and the cavities 

 of the hollow gonads a greatly reduced ccelome from the 

 epithelium of which the sex-cells are produced. 



The nervous system is formed on quite the same plan as 

 that of Polygordius, consisting of a dorsal brain (r) united 

 by oesophageal connectives to a ventral nerve-cord ( V. 

 Nv.Cd). In the cord, however, the nerve-cells, instead of 

 being evenly distributed, are aggregated into little enlarge- 

 ments or ganglia (Gn), of which there is primatively a pair 

 to each metamere, the number being reduced in the adult 

 by concrescence. The portions of the ventral nerve-cord 

 between the ganglia consist of nerve-fibres only, and are 

 called connectives. In the embryo the nervous system is, 

 as in Polygordius, in direct connection with the epidermis, 

 but in the adult it has sunk inwards so as to be entirely 

 surrounded by mesoderm. 



A striking feature in the histology of the Crayfish, and 

 one in which it agrees with the vast majority of Arthropoda, 

 is the entire absence of cilia. Another peculiarity also 

 shared by the greater part of the phylum is that the sperms 

 are non-motile. 



The laid eggs become attached to the swimmerets of the 

 mother, and in this situation undergo their development. In 

 the fresh-water crayfish the young is hatched in a condition 

 closely resembling the adult, but in the lobster and the sea- 

 crayfish there is a metamorphosis. 



