98 LIFE IN THE SEA [PART I 



they disappear from the plankton. The beautiful Erictheus larva 

 of the Stomatopod Squilla appears in July and August. In July 

 the sea often becomes brilliantly phosphorescent and the tow-net 

 shews that this phenomenon is due, as a rule, to the protozoon 

 Noctiluca, an organism which is the usual cause of phosphorescence 

 in the sea off the west coast of England. 



The summer months are also characterised by the appearance 

 of two series of organisms which are an intolerable nuisance to 

 the fishermen and naturalists, Peculiar algae {Tetraspora and 

 Halosphaei^a) which secrete mucoid capsules, appear in great 

 abundance, clogging up the pores of the tow-nets, and drying on 

 the trawl nets, producing an unpleasant odour. Great swarms of 

 medusae float on the surface of the sea. At first Aurelia is the 

 common one, but later on the large " cabbage-bleb " (Rhizostoma) 

 makes its appearance. One may sail for miles through a swarm 

 of Aurelia so densely packed together that the sea has a uniform 

 reddish colour. When the trawl is hauled it is full of this jelly- 

 fish and great labour is often required to clear it, and when at 

 length it is brought on deck much shovelling is necessary before 

 the slobbery mass of Aurelia is thrown overboard. Large Rhizo- 

 stomae are often seen drifting about in the water, each with a little 

 crowd of gadoid fishes of small size sheltering underneath the 

 bells, doubtless seeking the shade and also protection from enemies, 

 to whom the stinging jelly-fish is a thing to be avoided. 



During the summer months the diatoms have not been so 

 abundant as formerly, though they are always present to some 

 extent. Rhizosolenia is often abundant, being perhaps more 

 characteristic of the summer months than the other diatoms 

 mentioned. With the autumn and winter months the diatoms 

 begin to flourish again. A second period of increased multiplication 

 occurs and from then to November they again become abundant. 

 We shall see that this second maximum, like the first one, depends 

 on the accumulation of dissolved food matter in the sea, perhaps 

 ultimately on the reduction of the temperature. 



Towards the end of the year the plankton as a whole falls ofi" 

 both in quantity and variety. The larval forms have mostly 

 become metamorphosed and have adopted bottom-living habits or 

 have taken to a free-swimming life in the sea as active animals. 



