THE COPEPODA 



73 



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

 with the morphology of the two groups. 



Morphology of Eucopepoda. 



Apart from the parasitic forms, which exhibit an endless 

 variety of modifications, the Eucopepoda present a consideraljle 

 nnifoi-mity in general shape. Tiie 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, hoAvever, 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 the line of articulation 

 defininer the first free somite 



segmentation of the body in Gymnoplea and 

 Podoplea. A, outline of the body of female 

 I'aliniKs (Gymnoplea), dorsal view. 15, abdomen 

 of male CalaiiKS. C, outline of female Cti'-lojis 

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



runs in front of the maxillipeds, thoracic somites (commonly reckoned as first, 

 1 . . ., , , , second, and fifth respectively); 1 



+2, first and 

 lecond abdominal somites, coalesced in the female 

 sex in both instinces ; t, telson ; /, catidal furca. 

 * marks the jjosition of the movable articulation 

 ttetween the anterior and posterior divisions of 

 the body. 



and it is possible that the 

 somite bearing these appendages 

 should be regarded as coalesced 

 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 

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



