214 THE CRUSTACEA 



to the adults. In many parasites, however, a certain amount of 

 metamorphosis occurs. In the Cymothoinae, for instance, the 

 young are free-swimming, but the adults lose the natatory setae of 

 the pleopods and undergo considerable changes of shape in becoming 

 permanently attached to the host. In this case the life-history is 

 complicated by the fact that each individual passes through stages 

 in which it presents the characters, first of the male, and afterwards 

 of the female sex. The remarkable changes in the structure of the 

 mouth-parts, which are accompanied by considerable changes of 

 general shape, in the Gnathiidae have already been described. 



It is in the Epicaridea, however, that the changes during 

 development are most profound. In spite of the great diversity of 

 structure among the adults, the natural character of this group is 

 rendered evident by the uniformity of the larval stages throughout 

 all the families. In the first or epicarid stage ("stade epicaridien," 

 Bonnier) (Fig. 129, B) the body is short and broad and strongly 

 convex dorsally, with seven thoracic and six abdominal somitet 

 distinct; the antennules are very short, the antennae longer and 

 used in swimming, and both are sparingly provided with sensory 

 filaments ; six pairs of thoracic legs are present, all, except some- 

 times the posterior pairs, strongly subch elate ; the pleopods are uni- 

 or biramous, with natatory setae ; the uropods are usually biramous 

 and styliform ; the telsonic segment is generally produced into an 

 " anal tube " ; eyes are usually present but imperfectly developed. 

 In the last larval or cryptoniscan stage ("stade cryptoniscien," 

 Giard and Bonnier) the body is elongated ; the antennules are often 

 biramous, with numerous sensory filaments ; seven pairs of thoracic 

 legs are present, with coxal plates, and at least the anterior pairs are 

 subchelate; there is no anal tube; the eyes are well developed, some- 

 times very large. According to Sars, whose views have recently 

 received support from some experiments by Caullery, a third larval 

 stage intervenes, in some if not in all cases, between the two just 

 described. In this stage, formerly described as a distinct genus 

 under the name Microniscus, F. Miiller, the larva is temporarily 

 parasitic on pelagic Copepoda (Gymnoplea). A certain amount of 

 retrogressive metamorphosis takes place, the appendages are imper- 

 fectly segmented, the muscles appear to degenerate, and the pleopods 

 lose their natatory setae. Later, the larva assumes the cryptoniscan 

 form and leaves the Copepod to seek a second host. In those 

 families of the Epicaridea grouped together in the tribe Crypto- 

 niscina, the male becomes sexually mature in the cryptoniscan stage, 

 while the adult female, which, in some if not in all cases, is the 

 same individual in a later stage of development, becomes variously 

 degenerate and may lose all appendages and even all traces of 

 segmentation. In the tribe Bopyrina, where the occurrence of 

 hermaphroditism is doubtful, both sexes pass beyond the crypto- 



