22 THE CRUSTACEA 



or capsules in which the sperms are transferred to the female. 

 The terminal portion of the male duct is sometimes protrusible, 

 and acts as an intromittent organ, or this function may be discharged 

 by some of the appendages. 



The position of the genital apertures varies greatly in the different 

 groups. The most anterior position is found in the Cirripedia, 

 where the oviducts open on the first trunk-somite, while in certain 

 Branchiopoda (Polyartemia), on the other hand, the genital apertures 

 lie behind the nineteenth trunk-somite. It is characteristic of the 

 Malacostraca that the position of the genital apertures is different 

 in the two sexes, the female openings being on the sixth and the 

 male openings on the eighth trunk-somite. In all the other groups, 

 with exception of the hermaphrodite Cirripedia, the position is the 

 same in the two sexes. 



While very few Crustacea are viviparous in the sense that the 

 eggs are retained within the body until hatching takes place (some 

 Branchiopoda), the great majority carry the eggs in some way or 

 other after their extrusion. The various devices by which this is 

 accomplished will be described in dealing with the different 

 groups ; but it may be mentioned here that a few cases are known 

 (Cladocera, terrestrial Isopoda) in which the developing embryos 

 are nourished by a special secretion while in the brood-chamber of 

 the mother. 



EMBRYOLOGY. 



The majority of the Crustacea leave the egg in a form more or 

 less different from that of the adult, and pass through a series of 

 free-swimming larval stages, but there are many cases of direct 

 development in which the newly hatched young resemble the 

 parent in general structure. The relative size of the egg is greater 

 in those forms which develop without metamorphosis, except where 

 other means exist for the nourishment of the developing embryos. 



The details of the early stages of development differ consider- 

 ably within the limits of the class. They are chiefly of interest, 

 however, from the point of view of general embryology rather than 

 from that of the special student of the Crustacea, and can only be 

 very briefly referred to here. An admirable summary of the whole 

 subject will be found in Korschelt and Heider's Text-book of the 

 Embryology of Invertebrates. 



Segmentation is usually of the superficial or centrolecithal type, 

 or some modification thereof. The hypoblast is formed either by a 

 definite invagination or by the immigration of isolated cells which 

 wander through the yolk as " vitellophags " and later become 

 associated to form the mesenteron, or by some combination of the 

 two methods. The blastopore generally occupies a position corre- 

 sponding to the posterior end of the body. The mesoblast of the 



