FEEDING OF MARINE ANIMALS 197 



vores, and those which feed indiscriminately upon any kind 

 of food ; but owing to our lack of knowledge concerning the 

 food of many marine beasts, it is perhaps better to divide up 

 marine animals according to the method by which they 

 feed rather than the particular substances which they eat. 

 Thus we may divide animals into those which possess the 

 means for feeding on fine particles, on large masses or 

 living prey, and finally for sucking in fluid food. To these 

 must be added the parasites which prey upon other animals 

 and those remarkable cases of intimate union between two 

 animals or between an animal and a plant, the two being 

 entirely or partly dependent one upon the other, a condition 

 known as " symbiosis," from two Greek words meaning 

 " together " and " life." 



Invertebrate Animals living on Fine Food 



The animals which feed on microscopic plants and animals 

 or on fine particles in suspension in the sea are usually 

 either sessile creatures attached to the bottom, animals 

 which cannot go in search of prey but have to take what 

 they can from the water which flows past them, or else 

 they are small animals of the plankton. Many of the 

 mechanisms with which they are provided for collecting 

 their finely divided food are extremely complicated, and the 

 mode of feeding in some of these animals is the most 

 elaborate found in the animal kingdom. 



A very large number of these animals create a stream of 

 water by means of tiny, rapidly moving hairs, known as 

 '* cilia " or " flagella," the food particles being sieved out 

 and swallowed. The simplest mechanism of this type is 

 found among the sponges which are honeycombed with a 

 series of fine canals, through which water passes to be later 

 expelled by a large central opening or " osculum," all food 

 particles in the water having been seized and absorbed 



