GENERAL NOTES ON CRUSTACEANS. 



277 



is an organised method of getting rid of some waste 

 products. 



There are many peculiarities connected with reproduc- 

 tion—thus parthenogenesis for prolonged periods is common 

 among " water-fleas " ; hermaphroditism is frequent, occur- 

 ring, for example, in barnacles, acorn-shells, etc., and it is 

 often complicated by the 

 simultaneous existence of 

 " pigmy " complemental 

 males. When separate 

 the two sexes are often 

 very diverse. The 



spermatozoa are usually 

 exceptional in being very 

 slightly motile. In both 

 sexes some appendages 

 are often modified for 

 copulation or for carrying 

 the eggs. 



Development. — The 

 ova of most Crustacea 

 show considerable simil- 

 arity to those of Astacus, 

 and the segmentation is 

 typically of the kind 

 already described. But 

 while this is the most 

 typical case for Crust- 

 acean, and, indeed, for 

 Arthropod development, 

 it is possible, within the 

 limits of the class Crust- 

 acea, to trace out a com- 

 plete series, in which the first term is a segmentation of the 

 complete and equal type, like that of a worm, and the last 

 the purely peripheral. In the same way, though gastrula- 

 tion is usually much disguised, there are many modes, from 

 an invagination of the simplest embolic type {Lucifer), and 

 through the condition described for Astacus, to the forma- 

 tion of endoderm by the ingrowth of a solid plug of cells 

 (Arthrostraca, etc.). 



Fig. 119. — Zoasa of common shore crab 

 (Carcinus meenas). — After Faxon. The 

 appendages are numbered ; c. , gills ; 

 i., alimentary canal. 



