DISCUSSION OF SPECIES 



197 



tows ran into the hundreds. It was taken in 20 nocturnal and 

 65 diurnal surface tows, 129 50-meter tows, and 120 100-meter 

 tows, as well as in the vertical tow from 1000 meters, station 

 64. As this record would indicate, it is more or less in- 

 different to light and was often found equally distributed in 

 all three tows at the same time. In view of this Carnegie 

 record, it seems strange that the species did not appear at all 

 in the Siboga plankton. 



Oithona spinirostris Claus 



\Oithoiia spinirostris Claus, Die freilebenden Copcpoden, p. 

 105, pi. II, 1863.] 



This species was also well distributed in both oceans, but 

 not so widely or so abundantly as similis. It was present in 

 3 nocturnal and 8 diurnal surface tows, 77 50-meter tows, 84 

 loo-meter tows, and at station 64 in the vertical tow from 

 1000 meters. This record shows a definite aversion to light, 

 and some preference for the lOo-meter tow; the surface 

 records are very small as compared with the two deeper 

 tows. Farran (igoSrt, p. 500) gave to this species the new 

 name athintica, but Sars (1913, P- 6) has clearly shown that 

 it is the same as the species designated spinirostris by Claus 

 ( 1863, p. 105), and the latter name must be retained. 



Oithona vivida Farran 



[Oithona viuida Farran, Proc. Zool. Snc. London, 1913, p. 1S3, 

 pi. 27, figs. 1-8, 1913.] 



Three females of this species were obtained at station i," 

 in the eastern tropical Pacific, i specimen in the 50-meter 

 tow and 2 in the 100-meter tow. It was originally obtained 

 by Farran (1913, p. 183) from the vicinity of Christmas 

 Island in the Indian Ocean, and hitherto has not been re- 

 ported in other collections. It is thus a rare species. These 3 

 specimens showed an aversion to light, whereas those de- 

 scribed by Farran were taken in shallow water near the 

 shore; the depth of the tow was not given. 



Genus OITHONINA Sars, 19 13 



Oithonina nana (Giesbrecht) 



[Oithoini nana Giesbrecht, Fauna und Flora des Golfes von 

 Neapel, vol. ig, pp. 538, 549, pi. 4, fig. S; pi. 34, figs, 10, 11, 

 20, 24; pi. 44, figs. 4, 6, 1892.] 



This species was widely scattered in both oceans, but with 

 numerous gaps in the distribution. It was taken in 6 noc- 

 turnal and 15 diurnal surface tows, 32 50-meter tows, and 

 36 loo-meter tows. It is thus probably somewhat indifferent 

 to ordinary light, but becomes more or less negative to strong 

 light, and retires from the surface. It is included by many 

 authors in the genus Oithona, but Sars (1913, p. 5) has 

 given good reasons for considering it a separate genus. 



Genus ONCAEA Philippi, 1S43 



Like Oithona, this is a genus of cyclopoid copcpods, all of 

 whose species are small, but are inclined to corpulence rather 



than slenderness. The Caribbean region of the Atlantic and 

 the eastern tropical and southeastern temperate regions of 

 the Pacific contained the largest number of species as well as 

 individuals. The Carnegie records show a definite preference 

 for the deeper tows rather than the surface. Stations 35 to 65 

 yielded from one to ten species apiece, with an average of 

 six, and it is worthy of note that all these stations are in that 

 region of the Pacific where the oceanic currents are most 

 numerous and active. 



Chart 13. Daytime vertical distribution of species of Oncaca: 

 (r) anglica, (2) conifera, (3) curta, (4) ciirvata, (5) dcntipcs, 

 (6) media, (7) mediterranca, (8) minuta, (9) notopa, (10) 

 ornata, (11) similis, (12) subtilis, (13) tcnclla, (14) venusta. One 

 species was confined to the surface, one to the loo-meter tow, and 

 one to the two deeper tows. Each of the remaining eleven species 

 was least abundant at the surface and more abundant in the two 

 deeper tows. Four of them were most abundant in the 100-meter 

 tow, three were most abundant in the 50-meter tow, and the 

 other four were evenly divided between the two deeper tows. 

 Attempts have been made by various authors, beginning with 

 Johannes Miiller, to divide the plankton into horizontal zones 

 according to the depth at which it is found. In any such division 

 the species of this genus would probably be regarded as belonging 

 to the surface zone, since it is doubtful if any of them descend 

 much below the upper 100 meters, pracucally the surface when 

 compared with the depths of the ocean. Yet the majority of the 

 species show within their limited habitat a more or less well de- 

 fined aversion to the actual surface layer and a preference for the 

 two greater depths. [O. ornata was taken only at night. — Ed.] 



Oncaea anglica Brady 



\Oncaca anglica Brady, Trans. Nat. Hist. Soc. Northumber- 

 land and Durham, n. s., vol. i, p. 220. pi. 6, figs. 1-9, 1905.] 



Not found in the Atlantic plankton, and confined to the 

 first two stations in the tropical Pacific after passing through 

 the Panama Canal. Two females were taken in the 50-meter 

 tow at station 35 and i in the 100-meter tow at station 36. 

 TJhese 3 specimens were evidently negative to light, but the 

 number is too small to warrant a general statement. The 

 species was originally obtained from the North Atlantic, and 

 this is the first record from the Pacific. 



