58 



COPEPODA 



Introductory Remarks. The material which I have examined of this species has been very 

 big, as it is one of the most abundant and widely distributed plancton-copepods of the northern seas; 

 it was therefore quite necessary for me to try to solve the question, whether different or only a single 

 species alike to Pseitdocalanus minutus exist; unfortunately the result of my investigations is not quite 

 satisfactory. Sars has been the first, who has pointed out that different types exist, and taking the 

 scanty material into consideration he was quite right in establishing the three species Ps. elongatus 

 Boeck, Ps. major G. O. S. and Pseitdocalanus gracilis G. O. S. No naturalist has later on tried to solve 

 the question, scarcely touched it 1 ; most of them have examined specimens from southern regions and 



have scarcely been wrong in 

 referring them to the Ps. elon- 

 gatus Boeck (sensu strictu). 

 According to Sars the 3 species 

 are characterized in the fol- 

 lowing manner. 



1) Ps. major G. O. S. Aver- 

 age length of $ 2-4 mm. The 

 head is only slightly produced 

 anteriorly; the abdomen is half 

 as long as the cephalothorax. 

 The antennulae extend almost 

 to the middle of the abdomen. 



2) Ps. elongatus Boeck (sens, 

 strict). The average length 

 is 1-4 mm. The head is only 



slightly produced anteriorly; the urosome somewhat exceeding half the length of the anterior division. 

 The antennulae scarcely reach beyond the second caudal somite. 



3) Ps. gracilis G. O. S. Average length 1-65 mm. The body is more slender and the head anteriorly 

 distinctly produced; the urosome scarcely exceeds half the length of the anterior division; the caudal 

 rami are comparatively narrower and more divergent. The antennulae reach to the end of the third 

 caudal somite. Legs considerably more slender than in the typical species, with both rami very narrow. 



It will be noticed that in most characters, except in size, the Ps. gracilis differs almost equally 

 from the two other forms. The character found in the size and the length of the urosome is at 

 least of very relative value, as even from the same station (Ing. Nr. 31) 2 the following was observed: 

 Spec. Nr. 1 2*3 mm. with ant. division 2*5 x urosome, Nr. 2 r8 mm. ant. div. 2 - 5 x uros. (head of 

 gracilis type); Nr. 3 i - 8 mm. ant. div. 2 x uros. (head elong. type); Nr. 4 17 mm. ant. divis. 

 2-5 x uros., Nr. 5 1-5 mm. ant. divis. 2*4 x urosome; Nr. 6 1-4 mm. ant. divis. 2 - i x urosome; Nr. 7 1-2 mm. 

 ant. divis. 2*3 x urosome. The relation between the length of the urosome and anterior portion has 



Text- fig. 13. Psendocalanus minutus Kr. 

 fQ. Head from the left X no. b — c. Y9 — d (stage V) Abdomen X 59- 



Yd 1 (stage V) Pes V X 150. e. YJ (stage IV) Abdomen X 59- 



f. Yd" (stage IV) Pes V X 150. 



1 With the exception of Dam as and Koefoed, who have often been in difficulty about the identification of the 

 specimens and p. 406 write "Ces deux especes, distinguees par Sars, ne sont peut-etre que des variations d'une seule forme". 



2 N. B. from this station was found copepodite (St. V) 17 mm. and cop. (St. IV) 1*4 mm. long. 



