GENERAL NOTES ON CRUSTACEANS. 313 



Decapods there are three pairs ; a process of rapid growth 

 and branching converts these into the compact digestive 

 gland of the adult Decapods. In connection with the 

 posterior end of the mid-gut in Amphipods and some 

 others, there is a pair of blind tubes functioning as excretory 

 organs, and presenting an interesting similarity to the 

 Malpighian tubes of insects, which, however, are in con- 

 nection with the hind-gut. The body cavity is never large, 

 being mainly filled up with muscles and organs, and, as in 

 Arthropods in general, the true ccelom is virtually absent. 

 In the blood, haemocyanin is the commonest pigment, but 

 is not universal. Respiration 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 Entomos- 

 traca 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 possibly modified nephridia. 



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., ancl it is 

 often complicated by the simultaneous existence of " pigmy " 

 complemental males. When separate, the two sexes are 

 often very diverse. The spermatozoa are often exceptional in 

 being very slightly motile. 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 



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