REPRODUCTION. LARVAE. 355 



stated, that they are always motionless in Crustacea ; for those 

 of Polyphemus, Cirripedes and Ostracods have been observed 

 actively mobile. The non-mobile condition finds a parallel in the 

 Chilognatha and in some Arachnids. In Decapods fertilisation 

 is effected, according to Koltzoff, by the action of an explosive 

 capsule which is carried by the spermatozoon (cf. p. 525). 



In many groups of Crustacea the spermatozoa are encased 

 in spermatophores, secreted by the terminal portions of the 

 male ducts. 



The remarkable manner in which the eggs of the Cladocera 

 are nourished at the expense of neighbouring germ cells is 

 alluded to in the description of that group (p. 379). 



In the Entomostraca the genital ducts of both sexes usually 

 terminate at a segment lying at the limit between thorax and 

 abdomen whatever the numerical position of this segment may 

 be ; though the Cirripedes are exceptional in that the oviducts 

 open on the basal segment of the first thoracic appendage. 

 In the Malacostraca the positions of the genital openings are 

 fixed, like the number of the segments, the oviducts opening 

 on the 6th, the vasa deferentia on the 8th thoracic segment, 

 either on the base of the appendage or near by on the sternum. 

 It follows, on the view that the genital ducts are derived 

 from segmental organs, that in the Malacostraca the segmental 

 organs of different segments subserve the transmission of the 

 reproductive products in the two sexes. 



In many Copepods paired or single sperm passages are found 

 in the female leading to the receptaculum seminis and distinct 

 from the direct orifices of the oviducts. To these openings (or 

 opening) the spermatophores are attached by the male. The 

 remarkable birth-aperture described by Schobl in woodlice is 

 referred to in the section dealing with the Isopoda (p. 483). 



The secondary sexual characters in the shape of sense organs 

 and prehensile modifications of the limbs, together with the 

 many remarkable forms of sexual dimorphism found among the 

 Crustacea are described under the several subdivisions. 



Larval histories. One of the most interesting features of the 

 Crustacea is the occurrence in all the chief groups, however 

 diverse the forms of the adult may be, of a larva with cer- 

 tain constant characters, the Nauplius (Fig. 242). The name 

 was originally given by 0. F. Miiller towards the end of the 



