THE COPEPODA 87 



The brood- pouches of the Ascidicolidae have already been 

 alluded to. 



The testis, like the ovary, may be paired or single, the former 

 condition occurring chiefly among the parasitic forms. The vas 

 deferens is sometimes developed only on one side of the body. 

 This arrangement is universal among the Gymnoplea and is found 

 also in some Harpacticidae. Three regions are distinguished in the 

 vas deferens, which, however, are not sharply defined from each 

 other. A narrow proximal portion is followed by a wider part in 

 which the spermatozoa accumulate and become surrounded by a 

 layer of secretion giving rise to the sheath of the spermatophore 

 and a widened terminal part in which the development of the 

 spermatophore is completed. 



The possession of definite spermatophores seems to be a universal 

 character of the Eucopepoda, distinguishing them from all the 

 other " Entomostracan " orders. The spermatophores may be 

 globular, pyriform, or, commonly, sausage-shaped, and consist of a 

 firm cuticular (not chitinous) investment enclosing a mass of 

 spermatozoa together with a substance which by its expansion 

 serves to expel the spermatozoa. In addition, the spermatophore 

 contains a coagulable secretion which is expelled before the 

 spermatozoa and forms a sheath surrounding them within the 

 female spermatheca. Externally the " neck " of the spermatophore 

 is surrounded by a mass of a cementing substance secreted in 

 the terminal portion of the vas deferens for attachment to the 

 copulatory aperture of the female. In the Gymnoplea the last pair 

 of thoracic feet of the male are modified to form an apparatus by 

 which the spermatophores are transferred to the female. In the 

 other Eucopepoda special copulatory appendages are absent. 



DEVELOPMENT OF EUCOPEPODA. 



The majority of the Eucopepoda hatch in the form of a very 

 typical Xan^ilms larva, though many parasitic forms reach a later stage 

 of development within the egg. The adult stage is reached by a very 

 gradual metamorphosis, the most marked change of shape occurring 

 (in the free-living forms) in the transition from the last metanauplius 

 to the first " Copepodid stage." In the parasitic forms great changes 

 occur in the later stages, some of Avhich arc described below. 



The youngest nauplius stages (Fig. 6, p. 11, and Fig. 49, A) 

 have an oval unsegmented body from which the dorsal shield is 

 not yet defined, a large labrum, and the usual three pairs of 

 appendages, the second pair (antennae) bearing a masticatory 

 process, while the third pair (mandibles) are often without such a 

 process at this stage. A large unpaired eye and a pair of antennal 

 glands (which later degenerate) are present. 



