CH. Ill] LIFE IN THE SEA 73 



which are planktonic animals throughout their lives. Conspicuous 

 among these are the arrow worms (Sagitta, Spadella, &c.) which are 

 ubiquitous and sometimes appear in the plankton to the exclusion 

 of all other kinds of organisms. Some of the true worms (Chae- 

 topoda) are pelagic dwellers, as for instance the beautiful creatures 

 Ghaetopterus and Tomopte^^is — and these in some seas are abundant. 

 Then we have many worms which produce pelagic larvae, which for 

 a time are conspicuous members of the plankton but are of little 

 importance in the general economy of the sea. 



\^ The Echinoderms (the starfishes, sea-urchins and sea-cucumbers) 

 are a numerous and varied marine " phylum." They are exclusively 

 marine and none is an inhabitant of fresh water. All live at the 

 sea bottom and only one or two, such as the Comatulidae, and 

 sometimes the common starfish (which occasionally swims in the 

 manner of a medusa by the rhythmical contractions of its arms), 

 appear exceptionally in the nekton. But the larvae of some of the 

 sea-urchins (as for instance Echinocardiuni cordatum) are among 

 the most beautiful of the forms which can be taken by the tow-net. 

 Many plutei and other larvae of echinoderms may be found 

 among the transitory plankton, but none is of importance in the 

 metabolism of the sea. 



The Coelenterates (jelly-fishes, zoophytes, corals, &c.) are very 

 important planktonic groups. They are animals in which there is 

 no distinct body cavity, that is the body wall is a structure which 

 consists of two layers only and encloses a single cavity which serves 

 as a stomach, alimentary canal and body cavity, and has a single 

 opening which is surrounded by a circlet of tentacles as in the 

 familiar case of the common sea-anemones. The latter coelenterates 

 are solitary animals, but the majority of the class live as colonies 

 as in the case of the zoophytes, in which we have a number of 

 individuals living in association connected together by a common 

 fiesh and all growing on a plant-like skeleton ; and the corals which 

 also live as colonies but which form massive limy skeletons which 

 build considerable reefs and other formations. In the case of these 

 colonial animals the group which is the unit, is formed by the 

 budding of an originally single " zoid," though sexual reproduction 

 of course occurs among the coelenterates. These animals produce 



