4 MARINE AND FISHERIES 



5 GEORGE v., A. 1915 



a relatively long body and a still longer muscular tail; but the latter, instead of 

 being continuous with the hinder end of the body, is inserted at the centre of the 

 body, at a right angle, like a handle of a hammer or pick-axe. It is more at home 

 in the open sea than in confined, inshore waters, though the specimens were per- 

 fectly healthy. This form is found both in the Arctic and in the Antarctic Oceans, 

 and is therefore described as being bipolar. 



Another characteristic Arctic Appendicularian, Oikopleura labradorensis , 

 was not observed at St. Andrew's. Both of these species extend their range in 

 the spring and summer, when the polar water spreads southwards; and at that 

 season they have been taken in the North Sea (H. Lohmann). 



The principal factor governing the distribution of the organisms of the Plank- 

 ton is the temperature of the sea; this is even more effectual than the salinity of 

 the water,* From the open sea where the salts are dissolved at a concentration 

 of 35 per cent, Fritillaria borealis is periodically transported to the brackish water 

 of the Baltic Sea with a salinity of 15 per cent. It is therefore not so surprising 

 as it appeared at first sight, to find this delicate form near the mouth of the St. 

 Croix river at St. Andrew's, more especially since the Appendicularians are known 

 to feed largely upon the Peridinians. 



In what has been said it is implied that the physiological aspect of the Plankton 

 is that which concerns the practical questions of nutrition and distribution. What 

 is known as its morphological aspect cannot be regarded as having any bearing 

 upon the fisheries, except in respect of the fundamental distinction between the 

 zooplankton and the phytoplankton. The true relationship of any planktonic 

 species have nothing to do with their food-value to other species. In this connection 

 the contrast between morphology and physiology is exactly analogous to that 

 which, as we have seen, can sometimes be drawn between observation and ex- 

 periment. In a complete scientific presentation of the subject it is impossible to 

 divorce the one from the other, especially if we desire to penetrate into the ob- 

 scure origins of the Plankton. 



It may therefore be of interest to recall that Haeckel looked upon the Ap- 

 pendicularians as representing the common stem-form or ancestral stock both 

 of sea-squirts (fixed Tunicata) and of fishes (Vertebrata) . It is worth while ex- 

 amining this opinion from the standpoint of the marine Plankton as a whole, 

 of which the Appendicularians are one of the most constant constituents. Haeckel's 

 view involves the assumption that they are primarily pelagic; and as this assumption 

 is the crux of the entire question, it. is certainly one which should be scrutinised 

 with the utmost circumspection. 



In dealing with this matter it should be borne in mind that adaptation is the 

 first consideration and that it is not necessary, at the outset of the discussion, to 

 dwell upon details of structure or life-history. In very many instances (e.g. the 

 pelagic Mollusca) it is usually taken for granted that the pelagic habit of the 

 organisms of the Plankton is a special adaptation from a bottom dwelling or 

 benthonic life to a surface-frequenting or planktonic life. 



* Carl Chun. Die Beziehungen zwischen dem arktischen und antarktischen Plankton. 

 Stuttgart, 1897. 



