CANADIAN FISHERIES EXPEDITION, 191Jrlo 



175 



subdivision of the earlier fourth segment. There is a similar sequence of segmenta- 

 tion in Tortanus. The development consists of three main periods : embryonic period 

 within the egg; nauplius or larval period comprising six stages; post-larval or copepodid 

 period comprising six stages, of which the sixth is the adult (see Lebour 1916). 



Table B. — Copepodid Stages of Calanus (Damas 1905). 



At least two other spefies. nf siiinll f^ize. must bo ro"-arded as important ^mirres of 

 food for young fishes in the gulf of St. Lawrence, viz, Pseudo calanus elongatus which 

 attains a length of 1-5 mm., and Tortanus discaudatus (ahout 2 mm.). The former 

 abounds also in the Norwegian sea, the latter is a characteristic Laurentian species. 

 When these forms and others which are commonly associated with them, e. g. Temora 

 longicornis (1-5 mm.) and Centro pages haniatus (1-5 mm.), occur in such numbers as 

 to overshadow the larger forms, they give the plankton a distinctive character both as 

 to colour and size, the dominant tone after preservation in formalin being a dusky 

 grey, and the individual dimensions aggregating to give the aspect of a microcalanoid 

 plankton. A microcalanoid plankton may also be brought about by the predominance 

 of the young stages of larger species, especially those of C. finmarchieus. When the 

 individuals are larger and the dominaiut colour, before and after preservation, is red, 

 owing to the quantity of red oil in their bodies, the plankton wears a megacalanoid 

 aspect. Such a sample may be composed in varying proportions of Calanus finmar- 

 chieus ( 3 to 5 mm.) C. hyperhoreus (3 to 7 mm.) and Euchaeta norvegica (3 to 8 mm). 

 Intermingled with these we find sometimes the white Metvidia longa which according to 

 Farran (1910) is the "most typically Arctic copepod of whose distribution there is 

 any accurate knowledge" (quoted by Bigelow, 1915, p. 292). The four last-named 

 species are as a general rule limited to the waters north of Cape Cod but C. finmar- 

 chieus ranges to the south of Nantucket, and Euchaeta norvegica was taken from 50-0 

 fathoms in lat. 40° N., long. 69° 29' W. (Bigelow, 1915). 



Giesbrecht tabulated the species of pelagic copepods under three leading regions: 

 species of the warm region (between 47° N. and 44° S.) ; species of the northern cold 

 region; species of the southern cold region. He showed that the copepod farmas on 

 opposite sides of the American continent are more nearly related than those of the 

 three hydrographical regions named above. Thus the main faunistic differences ap- 

 pear in following the distribution from the equator to the poles or from the poles to 

 the equator, not from the eastern to the western hemispheres. 



Calanus finmarchieus is not only the commonest copepod in eastern Canadian 

 waters and in the North Atlantic coastwise waters generally, but it occurs more abun- 

 dantV" than any other form in the San Diego region where its daily vertical migrations 

 have been studied by C. O. Esterly (1911). According to O. O. Sars (1901) both 

 C. finmarchieus and C. hyperhoreus extend tbroiighout the Polar Sea from Greenland 

 in the west to Behring Strait in the east. He adds that the former species is equally 

 devoured by herring and mackerel and " in some cases, as stated by Prof. "Robert 

 Collett, forms almost the exclusive nourishment of one of our greatest whales, Balae- 



