8ALPA IN RELATION TO EVOLUTION OF LIFE. 133 



which hunt and destroy each other, but there is no group of 

 marine animals which corresponds to the herbivora and rodents 

 and plant-eating birds and insects of the land. The pelagic 

 copepods are, of all the marine Metazoa, the ones whose place in 

 the economy of nature is most like that of the terrestrial plant- 

 eaters. They swarm in innumerable multitudes at the surface of 

 the ocean, and also below it down to a depth of a mile or more, 

 and they furnish the chief food for most young fishes, and for 

 great armies of herrings and pteropods and jelly-fishes and siphono- 

 phores, and for most pelagic larvae. 



There are plant-eating molluscs and echinoderms and annelids 

 in the ocean, but not in sufficient numbers to play any conspicuous 

 part in its economy, and the copepods are the only plant-eaters 

 which exist in sufficient numbers to be compared with those of the 

 land, and the food of the copepods is only partially vegetable, for 

 they devour microscopic animals as well as microscopic plants, and 

 probably to an equal amount. 



The group Crustacea as a whole is a carnivorous one, however, 

 for while a few subsist on algse, their number is inconsiderable. 

 Others chew the mud of the bottom and extract its organic matter, 

 but this is chiefly animal and consists of foraminifera and rhizo- 

 pods and infusoria. 



The molluscs as a whole are carnivorous, and while there are 

 many exceptions, such as the nudibranchs for example, many 

 nudibranchs feed on hydroids. 



The cephalopods and pteropods and heteropods and many of 

 the gasteropods pursue and destroy their prey, and other gastero- 

 pods are scavengers, while the lamellibranchs gather up the micro- 

 scopic organisms which are drawn into their gills with the water. 



The majority of the worms and echinoderms are animal-feeders. 

 Some of them, like the common starfish, are actively predaceous ; 

 others, like the crinoids, gather up microscopic organisms from 

 the water ; others, such as most holothurians, eat the mud of the 

 bottom and digest out of it the foraminifera and small molluscs 

 and annelids and Crustacea which it contains, while others, such as 

 the sea-urchins of the coral reefs, grind away and swallow the 

 living coral. The universal presence of a poisoning apparatus in 

 the coelenterates shows that the food of this great and important 



