THE COPEPODA 



73 



from the Eucopepoda, it will be more convenient to deal separately 

 with the morphology of the two groups. 



A. 



MORPHOLOGY OF EUCOPEPODA. 



Apart from the parasitic forms, which exhibit an endless 

 variety of modifications, the Eucopepoda present a considerable 

 'uniformity in general shape. The body is divided into two 

 regions, of which the anterior is sub -cylindrical or flattened, 

 and is more or less sharply marked off from the posterior 

 region, which is usually much narrower and terminates in a well- 

 marked caudal furca. There are typically ten free segments (in- 

 cluding the telson) behind the head, and the limit between the two 

 regions is marked by a very movable articulation which falls either 

 between the fifth and sixth of these (Gymnoplea) or between the 

 fourth and fifth (Podoplea) (Fig. 40). Giesbrecht has suggested 

 that the limit between these 

 regions is really in the same 

 position in both cases, that the 

 fifth thoracic somite of the 

 Gymnoplea is unrepresented in 

 the Podoplea, and that the 

 so-called fifth thoracic somite 

 of the last-named group is a 

 " pre-genital " somite which is 

 suppressed in the Gymnoplea. 

 The evidence in favour of this 

 view, however, is very slight. 

 The head - region is usually 

 stated to include, in addition to 

 the primitive cephalic somites, 

 that corresponding to the 

 maxillip : eds or first post- 

 maxillary appendages. It 

 appears, however, that in some 

 larvae 

 defining the first free somite 



FIG. 40. 



Segmentation of the body in Gymnoplea and 



linA rvf nrHr-iilatirm Podoplea. A, outline of the body of female 

 UatlOn c a j (mils (Gymnoplea), dorsal view. B, abdomen 

 of male Calanus. C, outline of female Cydops 



. e , .... (Podoplea). ii, iii, vi, second, third, and sixth 



rUllS in front Of the maxilllpeds, thoracic somites (commonly reckoned as first, 

 A 'Ui * i. i ii, second, and fifth respectively); 1+2, first and 



aim It IS possible that the second abdominal somites, coalesced in the female 

 ;nmitp Vparinrr tViPCpaTmonrJarroc sex ' n both instances ; t, telson ; /, caudal furca. 



bearing these appendages . marks the ^^ O ' f { he mov ' a t; le articulation 



should be regarded as Coalesced b*t*wo the anterior and j>osterior divisions of 



not with the head-region but 



with the following somite. The next five somites bear swimming- 

 feet, and constitute what is commonly called the thoracic region. 

 It seems advisable, however, to extend the meaning of the term 

 tJtorax to include also the somite of the maxillipeds. The remaining 



