PHYLUM ARTHROPODA 551 



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

 the two examples. The median or nauplius-eye 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 of Decapoda are open sacs in 

 the basal segment of the same appendages, but in Schizopoda occur 

 as closed otocysts (Fig. 423, ot) in the eridopodites of the uropods. 



Reproduction. In most Crustacea the sexes are separate, but 

 hermaphroditism occurs in some Phyllopods, in nearly all Cirripedes, 

 and in certain parasitic Isopods (Cymotlwci}. In the latter case the 

 animals are protandrous, male organs being developed at first, and 

 female organs at a later stage. In many Cirripedia minute com- 

 plemented 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 the glands 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 perform movements after reaching the female 

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

 animal itself (Fig. 415, D). In many Entomostraca reproduction is 

 parthenogenetic. In Daphnia, for instance, the animal reproduces 

 throughout the summer by parthenogenetic summer eggs, which 

 develop rapidly in the brood-pouch (Fig. 314, 1, lyr.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 speci- 

 ally modified and forms the &phippvuwi 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 complete 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 

 which afterwards spreads over the whole yolk. But in most 



