THE COPEPODA 89 



approximating to their permanent form. The caudal furca is also 

 developed at this stage. There are typically four somites defined 

 in front of the unsegmented abdominal region in the first Copepodid 

 stage, and three pairs of swimming -feet. In each of the five 

 succeeding Copepodid stages a somite is added, giving, together 

 with the terminal segment or telson, the typical number of ten free 

 segments. In the majority of cases, however, as already mentioned, 

 the number of somites in the adult is reduced by coalescence or 

 suppression, with corresponding changes in the course of develop- 

 ment. The constriction which marks off the broad anterior from 

 the narrow posterior region falls, in the first Copepodid stage, 

 behind the third free somite. It is moved backward one somite at 

 each moult, the Podoplea reaching the final limitation of the regions 

 at the second and the Gymnoplea at the third Copepodid stage. 

 In the more primitive forms (Calanus) the development of the 

 limbs, like that of the somites, takes , place in regular order from 

 before backwards. In the more specialised forms, while the rudi- 

 ments of the limbs appear in this order, there is a tendency for the 

 anterior swimming-feet to outstrip in their development the maxillae 

 and maxillipeds, which remain for some time as rudimentary buds. 



LlFE-HlSTORY OF PARASITIC EUCOPEPODA. 



In no other group of Crustacea has parasitism led to such 

 diversity of structure and of life -history as in the Eucopepoda. 

 The parasitic habit of life has been adopted to a greater or less 

 degree by many very different families, and every transition is 

 found from the normal free-living types to those most completely 

 adapted to a parasitic life. 



In those Eucopepoda which, while parasitic, retain to some 

 extent the power of locomotion, the general structure of the adult 

 does not differ greatly from that of the free-living types and the 

 sexual dimorphism is not accentuated. Thus in the family Astero- 

 cheridae, which have, as a rule, completely suctorial mouth -parts 

 and are parasitic on various Invertebrata, most of the species are 

 capable of swimming and retain the general Copepod form. In 

 the Ascidicolidae, which live rather as commensals than as parasites 

 in the alimentary tract of Tunicata and Echinoderma, we find a 

 series leading from the little-modified forms (Notodefyhi/s, etc.) which 

 live in comparative freedom in the pharyngeal sac of the Tunicata, 

 and in which the adults of both sexes possess natatory thoracic limbs, 

 to those species which live in the stomach and intestine, and have 

 assiimed in the female an almost vermiform shape, with limbs adapted 

 to push their way through the contents of the alimentary canal of 

 the host. The male in most cases is free-swimming, at least in the 

 adult stage, and is correspondingly less modified in general form. 



