[currie] plankton COPEPODS 209 



and the cylinders disappear (Fig. 3). The pressure is greatest at 

 the tips of the rami as is shown by the different lengths of the cylinders 

 formed by the unfolding setae; those of the middle termina setae 

 being longer than the other terminal ones which in their turn are 

 longer than the one of the outer seta (Fig. 2). 



During this energy consuming process of exuviation the stored 

 supply of oil within the copepod, which is comparatively large at the 

 beginning of exuviation, gradually diminishes as it is being used for 

 food. The copepod does not dart about in the water, but remains 

 apparently motionless except for an occasional quiver as it frees itself 

 from its appendages. 



After the setse are quite unfolded the pressure exerted by the 

 tissues is so great that it forces the tips of the rami into the setse, 

 thus forming an inner cylinder at the base of each seta (Figs. 2-3). 

 These inner cylinders are also of definite lengths, those of the two 

 middle terminal setae being twice as long as the other two, and four 

 times as long as the outer one. If any seta of the caudal furca, except 

 the inner one, is pulled off, it slips off this inner cylinder formed by the 

 ramus at its base and leaves it exposed (Fig. 2). 



If the copepod is in a healthy active condition, initial attempts to 

 regenerate lost setae of the caudal rami are made and usually an irregu- 

 larly shaped, sometimes even bifurcated seta protrudes from the rim 

 of the cylinder (Fig. 4). When the setae do not wholly break away 

 from their bases but are injured or broken, regeneration is also at- 

 tempted (Fig. 27). 



Among all the individuals of C. finmarchictis III examined, none 

 was found exuviating to stage lY., but an excellent example of C. 

 hyperhoreus III. in this condition (Fig. 5) was obtained in a vertical 

 haul from "Prince" Sta. 3 on May 24th. This species differs from 

 finmarchicus in certain morphological features, but the two are so 

 closely related that the exuviation of both of them from one stage to 

 another takes place in a similar manner. 



Stage IV. In this stage there are five free thoracic segments 

 in the forebody, each with a pair of swimming feet, and three abdom- 

 inal segments in the urosome. The ramus externus (Re) and ramus 

 internus (Ri) of the fifth feet (p5), each consists of one joint. On the 

 proximal joint (Bi) of the stem of the fifth foot there are coxal teeth 

 like those of the adult but with a different distribution and varying 

 in number. 



Length variation. One hundred and forty eight specimens of 

 stage IV., taken in several hauls from "Prince" Sta. 3, were examined. 

 As shown by graph. I, instead of there being a continuous variation 

 in length, as appears to be the case in stage III, about a single mean. 



