208 THE ROYAL SOCIETY OF CANADA 



upon Eurytemora, Tortanus, Acartia, Pseudocalanjis and Metridia, 

 which will be detailed in their turn. 



A . Calanus finmarchicus . 



Stage III. In this stage there are five free thoracic segments in 

 the forebody, two segments in the urosome, four pairs of swimming 

 feet and the fifth pair (p. 5)^ rudimentary. 



Only twenty-three specimens of this stage were examined, all 

 being from "Prince" Sta. No. 3, June 28th, 1916, 10 fathom tow. 



Length Variation. The length varied from 1-5 mm. to 1-8 mm. 

 with an average of 1-615 mm. The variation is continuous without 

 any abrupt deviations which might be taken to indicate the existence 

 of two races at this stage. 



Exuviation. During the stages intervening between its successive 

 moults, a copepod is continually growing and acquiring the character- 

 istics of the following stage within its cuticle, which becomes stretched 

 to its limit of elasticity. The cuticle of the next stage, owing to its 

 larger surface, is wrinkled and crowded within the old one. When it 

 is ready to exuviate, the copepod first withdraws its eating and swim- 

 ming appendages and the caudal furca, beginning from the inner 

 lobes of the eating appendages, the inner rami of the swimming feet, 

 and the two rami of the caudal furca; the antennae are the last which 

 it withdraws. The new setae loosen themselves from the exuvia by 

 withdrawing within the body of the appendages as if they were being 

 turned inside out from the bases. This turning inside out process is 

 never completed for when the setse have been pressed in until their 

 tips are on a level with their bases the pressure created stretches the 

 internal cuticle until the exuvia reaches its limit of elasticity and 

 breaks on the top of the head and along the back. As soon as the 

 exuvia is broken the pressure is reversed. The setae are again stretched 

 out, in so doing pushing the exuvia away from the copepod. The setae 

 of the eating and swimming appendages are forced out by their con- 

 tents swelling and exerting pressure at the tips (Fig. I), whereas 

 those of the caudal furca are pushed out by the contents of the rami 

 swelling and exerting such pressure around the setae that the bases of 

 the setae are pushed out as cylinders about their protruding tips 

 (Fig. 2). These cylinders lengthen and as they lengthen the setae are 

 brought farther out from the body until they are quite stretched out 



^ List of abbreviations used in the text. 



p 5, Fifth swimming foot or feet; Li I, First inner lobe; Re Outer ramus or branch 

 of an appendage; Ri, Inner ramus; Bi, First coxal or basal joint; Se, Outer marginal 

 setae; St, Terminal setae. 



For explanation of figures see end of report. 



