298 PHYLUM ARTHROPODA. 



muscles and organs, and, as in Arthropods in general, the 

 true coelom is virtually absent. In the blood, heemocyanin 

 is the commonest pigment, but is not universal. Respira- 

 tion is carried on in many different ways. In the simple 

 forms it may be merely by the general surface, but in the 

 majority of cases, certain portions of the limbs, or outgrowths 

 of the limbs, constitute definite respiratory organs, often 

 specialised to form gills. In the excretory system the 

 numerous nephridia of Annelids are absent. The typical 

 excretory organs of the Entomostraca are the " shell-glands " 

 -paired coiled tubes opening on the second maxilla ; of 

 the Malacostraca, the antennary glands exemplified by the 

 green glands of the crayfish. The genital ducts are pro- 

 bably modified nephridia, and the fact that they open on 

 different segments in the two sexes, is regarded as evidence 

 of the former existence of a series of nephridia like those 

 of Annelids. The process of excretion in the Crustacea 

 is not well understood ; it is possible that shell-making 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 excep- 

 tional 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 con- 

 siderable similarity to those of Astacus, and the segmenta- 

 tion is typically of the kind already described. But while 

 this is the most typical case for Crustacean, and, indeed, 

 for Arthropod development, it is possible, within the limits 

 of the class Crustacea, to trace out a complete 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 gastrulation 

 is usually much disguised, there are many modes, from 

 an imagination of the simplest embolic type (Lucifer), and 

 through the condition described for Astacus, to the forma- 



