238 CRUSTACEA. 



participate in the degeneration of the body -segments as a result 

 of their parasitic life, and they further undergo arrest of growth, 

 so that an enormously large female is contrasted with a dwarf male. 

 This kind of heteromorphous development of the two sexes must 

 be regarded as an excessive adaptation to the different sexual 

 functions, which is rendered possible by parasitic life. 



The parasitic forms can be deduced from the free-living forms by 

 imagining that the latter have, in consequence of parasitism, under- 

 gone certain changes. Thus the parasitic Copepoda first passed 

 through Metanauplius and Cyclops stages to a stage approaching 

 the shape of the free-living form, and then, through a series of 

 further stages, attained parasitic deformation. The metamorphosis 

 of the parasitic Copepoda has thus been lengthened by the addition 

 of final parasitic stages. The first two series of stages, however, 

 seem to be correspondingly shortened. The larvae of the parasitic 

 Copepoda frequently do not hatch in the Nauplius form, but in 

 an advanced Metanauplius stage, or even in the Cyclops stage (Fig. 

 73, p. 148; Chondracanthus, Tracheliastes, Achtheres, Anchorella y 

 Brachiella, etc.). On the other hand, the metamorphosis may be 

 shortened by the suppression of the later Cyclops stages, since, in 

 cases of the most specialised parasitic forms, the very first Cyclops 

 stage passes at once into the parasitic form (Chondracanthus, 

 Lernaeopodidae). 



A further distinction between the metamorphosis of the parasitic 

 Copepoda and that of the free-living forms arises from the circum- 

 stance that, even in the larval condition, a sedentary manner of life 

 (on the gills of a host) is adopted, and that, in keeping with this, 

 there is a development of a peculiar attaching organ (the frontal 

 band of the larvae in the Caligidae, Lernaea, and Lernaeopoda) and 

 of resting stages with reduced limbs (so-called pupal stages). 



It would take us beyond the limits we have assigned to ourselves to give 

 a complete enumeration of the very scattered notices of individual larval forms 

 among the parasitic Copepoda, especially as there are still many gaps in the 

 observations made on the development of these forms. We must content 

 ourselves with selecting a few of the more important forms, of whose meta- 

 morphosis a more accurate knowledge has been obtained. We must here, in 

 the first place, separate those larvae which apparently are not provided with 

 tbe larval adhering apparatus (frontal band) from those in which such an organ 

 has been observed. 



In bhOM families in which the adult retains more or less of the body seg- 

 mentation of the free-living Cupupoda {e.g., Corycacidae and the Notodelphyidae, 

 which latter is placed among the Gnathostomata), the metamorphosis appears 

 ""' to differ essentially from that above described for free-living forms. In the 



