582 ZOOLOGY SECT. 



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. 466, ot) in the endo- 

 podites 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 discharging 

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

 directly into the gonoducts and so to the exterior. The gonads may 

 be simple or branched, and frequently there is more or less con- 

 crescence 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 Ostra- 

 coda they perform movements after reaching the female ducts. 

 In some Ostracoda they are about three times as long as the animal 

 itself (Fig. 457, D). In many Branchiopoda and Ostracoda reproduc- 

 tion is parthenogenetic. In Daphnia, for instance, the animal 

 reproduces throughout the summer by parthenogenetic summer eggs 

 which develop rapidly in the brood-pouch (Fig. 456, 1, br. p.). 

 In the autumn winter 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 ephippium 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 (Lucifer, Euphausia, and 

 others) segmentation is complete, and a hollow blastula is formed : 

 in others division of the nucleus into a number of daughter nuclei 

 is followed by their migration towards the surface, where, each 

 becoming surrounded by protoplasm, they form a layer of cells 

 (blastoderm) enclosing a central mass of yolk (centrolecithal egg 

 with superficial blastoderm) : in others, again, the egg is telolecithal, 

 and the protoplasm, accumulated at one pole, divides so as to form 

 a disc of cells which afterwards spreads over the whole yolk. 



Development is always accompanied by more or less metamor- 

 phosis. In most Branchiopoda the young is hatched in the form of 

 a nauplius (Fig. 436, A), and further changes are of the same char- 



