NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 33 



Stage, and called this stage a naupHus (A) ; while Wilson and Capart 

 record both a nauplius and a metanauplius. The matter is of no 

 particular importance for us in a study of the metamorphosis of the 

 species. Whatever the larva that hatches from the egg may be, it 

 moults into a free-swimming copepodid (C), Though the copepodid 

 is only about half a millimeter in length, it has the responsibility of 

 finding a flounder and of fixing itself to the gills of the fish, for which 

 latter purpose it is provided with strongly chelate second antennae. 

 Its hold on the gill, Sproston says, is never relinquished, and becomes 

 the anchorage of the parasite until the free-swimming adult stage is 

 reached. The gill filaments, however, are grasped also by the second 

 maxillae in order to bring the mouth parts into close contact with the 

 tissues on which the parasite feeds. 



When the copepodid moults the larva becomes a chalimus (fig. 

 12 D), but there is little change in form or structure. The chalimus, 

 however, in its first instar acquires an additional attachment on the 

 host in the form of a filament secreted by a gland in the head, which 

 is anchored in the gill by two diverging branches that penetrate into 

 punctures in the gill tissue. The rest of the secretion from the gland, 

 Sproston says, falls back on the head of the larva where it hardens 

 into a conical hood. The chalimus goes through four instars, and 

 with each moult but the last a new hood is formed while the old ones 

 remain, so that there are thus formed a set of overlapping caps corre- 

 sponding in number with the moults. The third instar of the chalimus, 

 to be identified as such by its three hoods, is illustrated at D of figure 

 12, redrawn from Sproston. The copepodid and the chalimus are 

 metamorphic larval forms adapted to their respective functions of 

 swimming and parasitic feeding. During its four instars the chalimus 

 gradually approaches the adult structure, which is attained at the 

 fourth moult after the copepodid stage. 



The adult male of Lernaeocera (fig. 12 E) leaves the old attachment 

 filament with the castoff chalimus cuticle hanging on the gill of the 

 flounder, and goes off in search of a female. The female (F), how- 

 ever, awaits the coming of a male before she relinquishes her hold on 

 the flounder. When the male finds a female still attached, mating 

 takes place ; two large spermatophores are inserted into the genital 

 ducts of the female and are eventually lodged in her lengthened genital 

 segment (F, Sphr). The female, still not sexually mature, then frees 

 herself from the flounder and swims away to look for her second 

 host, which should be a cod. On attaining a prospective victim, the 

 female fixes herself to the bases of the gills by her second antennae, 

 and now begins her metamorphosis into the final egg-producing stage. 



