THE COPEPODA 89 



approximating to their permanent form. The caudal furca is also 

 developed at this stage. Thei-e 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 tlie majority of cases, however, as already mentioned, 

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

 suppression, with coi-responding 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 Gynmoi^lea at the third Copepodid stage. 

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

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

 before back^vards. In the more specialised forms, while the rudi- 

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

 anterior SAvimming-feet to outstrip in their development the maxillae 

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



Life-History 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 pai-asitic 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 dimoiphism is not accentuated. Thus in the family Astero- 

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

 and are parasitic on various Invertebrata, most of the si)ecies are 

 capable of swimming and retain the general Copepod form. In 

 the Ascidicolidae, which live rather as commensals than as parasites 

 in the alimentarj' tract of Tunicata and Echinoderma, we find a 

 series leading from the little-modified forms {XotodcI]>Jn/.% etc.) which 

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

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

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

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

 to push their way through the contents of the alimentar}^ canal of 

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

 adult stage, and is coi'respondingly less modified in general form. 



