CH. iy] life in the sea 99 



The perennial plankton continues. But the summer visitors 

 among the copepods and other Crustacea have departed ; also the 

 large medusae and other coelenterates, the pelagic algae, and many 

 other permanent planktonic animals and plants have gone. The 

 ubiquitous copepods such as Paracalanus, Pseudocalanus, Acartia, 

 Centropages, Temora, Oithona, and others remain in restricted 

 quantity. The ctenophore Pleurohrachia, the pelagic worm Sagitta, 

 and the ascidian Oikopleura remain also. Ceratiinn is nearly 

 always present. But all are less abundant during the last month 

 of the year. Finally, at the beginning of the year the diatoms 

 again begin to increase attaining their maximum in the spring. 



There is thus a regular succession of organisms in the plankton 

 of a sea area. In that just described we see that the main features 

 of this succession are : — (1) relative scarcity at the beginning of 

 the year; (2) an outburst of diatom life in the late winter and 

 early spring ; (3) the appearance of countless myriads of fish and 

 invertebrate eggs and larvae in the spring and early summer ; 

 (4) a decrease in the abundance of the diatoms, and the gradual 

 disappearance, from the plankton, of the eggs and larvae ; (5) the 

 appearance of swarms of medusae and other coelenterates in the 

 summer ; (6) the reappearance of diatoms in the late summer and 

 autumn ; and (7) the scarcity of the plankton as the winter begins. 

 Throughout all there are a number of forms of life which remain 

 as permanent inhabitants of the area. 



Just in the same manner the fishes and the bottom-living 

 invertebrata have their phases of abundance and scarcity. In the 

 Irish Sea cod appear in the winter and spring and spawn in the sea 

 offshore. Hordes of whiting visit the offshore fishing-grounds in 

 the spring. Flounders and small plaice migrate from somewhere 

 to the shallow inshore waters in the spring. Soles appear in the 

 deeper waters off the land in the summer. In the summer and 

 autumn plaice visit certain fishing grounds for a time and hake 

 migrate into the deepest parts of the sea from the south. Fisher- 

 men are w^ell acquainted with these periodic appearances of fishes 

 and regulate their operations accordingly; and just as in the case 

 of the plankton there are ubiquitous fishes, such as the dab, 

 which are to be found everywhere and at any time of the year. 

 No doubt corresponding series of changes occur in the case 



" 7—2 



