74 LIFE IN THE SEA [PART I 



larvae which make an evanescent appearance in the plankton, 

 though they are, in their final phases, benthic organisms. Two 

 great groups of coelenterates are permanent and important 

 planktonic animals. The medusae are very conspicuous pelagic 

 organisms. Every one who has been to sea must have seen the 

 immense shoals of jelly-fishes which at times cover the surface of 

 the sea. I have sailed for several miles through a shoal oi Aurelia, 

 in which the separate animals were closely packed and were piled 

 on the top of each other completely discolouring the sea. Then 

 there are the large white " Cabbage-blebs " (Rhizostoma) which 

 are at times so numerous as to form a great nuisance to the 

 fishermen. The medusae swim by the contraction of their bells, 

 but their powers of movement are not great enough perhaps to 

 justify us in including them with the true nektic animals. 

 Another group of coelenterates, which are of great interest by 

 reason of the extreme beauty of some of their species, are the 

 Siphonophores, of which the best known example is the Portuguese 

 man-of-war. They are inhabitants of oceanic areas and generally 

 of warm water. 



^ The Ctenophores are very common planktonic organisms. 

 In our seas the " Marble-bleb " (Pleitrobrachia) is the only very 

 common species. This beautiful little creature must be familiar 

 to any one who has seen a shrimp trawl hauled in our seas in the 

 summer. It is very often present in the tow-nets though not 

 in very great numbers, as its size prevents this apparatus fi:om 

 catching it in abundance. In warmer seas the pelagic ctenophores 

 are beautiful and conspicuous objects of the plankton. 



^ The Protista are the most abundant of the planktonic 

 organisms and by far the most difficult to study. They are for 

 the most part animals of very small size and on this account they 

 are not represented in the catches of plankton made in the 

 ordinary tow-nets to the extent that they occur in the sea. No 

 silk net has yet been constructed which will take a representative 

 sample of these organisms from the sea. The protista are uni- 

 cellular plants and animals, or they are those anomalous forms 

 which fit with difficulty into any wide scheme of classification. 

 We can divide them into the Protozoa, which are single-celled 



