XI PHYLUM ARTHROPODA 597 



The sense-organs are mostly of the same character as those of 

 the two examples. The median or nauplius-ej'e always occurs in 

 the larva, and can frequently be shown to exist in the adult of 

 even the higher groups (Decapoda). The Cirripedia and many 

 parasitic Copepods are eyeless in the adult, as also are certain 

 subterranean Malacostraca. Olfactory setae occur, as a rule, on the 

 antennules, and the auditory organs (or statocysts) of Decapoda 

 are open sacs in the basal segment of the same appendages, but 

 in Mysidacea they occur as closed cysts (Fig. 459, ot) in the 

 endopodites of the uropods. 



Reproduction. — In most Crustacea the sexes are separate, but 

 hermaphroditism occurs in some Branchiopods, in nearly all Cirri- 

 pedes, and in certain parasitic Isopods (Cymothoa). In the latter 

 case the animals are protandrous, male organs being developed first, 

 and female organs at a later stage. In many Cirripedia minute 

 complemental males are found attached, like parasites, to the body 

 of the ordinary or hermaphrodite individual, the male organs of 

 which appear to be inadequate for the full discharge of the ferti- 

 lising function. Sexual dimorphism is almost universal, and 

 reaches its maximum in the parasitic Copepods and Isopods 

 already referred to. 



The gonads are always a single pair of hollow organs discharg- 

 ing their products into a central cavity or lumen, whence they 

 pass directly into the gonoducts and so to the exterior. The 

 gonads may be single or branched, and frequently there is more or 

 less concrescence between those of the right and left sides, as in 

 Astacus and Cyclops. The sperms vary greatly in form, and are 

 usually motionless : in Cirripedia, however, they are motile, and 

 in Ostracoda they perform movements after reaching the female 

 ducts. In some Ostracoda they are about three times as long as 

 the animal itself (Fig. 450, D). In many Branchiopoda and 

 Ostracoda reproduction is parthenogenetic. In Daphnia, for 

 instance, the animal reproduces throughout the summer by 

 parthenogenetic simimer eggs, which develop rapidly in the brood- 

 pouch (Fig. 449, 1, hr. p.). In the autumn lointer eggs are produced, 

 which are fertilised by the males : they pass into the brood-pouch, 

 a portion of which becomes specially modified and forms the 

 tpJdppium or saddle. At the next moult the ephippium is 

 detached and forms a sort of bivalved capsule in which the eggs 

 remain in an inactive state during the winter, developing in the 

 following spring. 



Development. — In some Crustacea segmentation is complete, 

 and a hollow blastula is formed : in others segmentation is 

 followed by an accumulation of yolk in the interior, resulting 

 in the formation of a superficial blastoderm, as in Astacus : 

 in others, again, the egg is telolecithal, and the protoplasm, 

 accumulated at one pole, divides so as to form a disc of cells 



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