1 84 THE BIOLOGY OF THE SEA-SHORE 



off quantities of the myriad diatoms, protophyta, protozoa, 

 and tiny multicellular animals in which the upper waters 

 of the sea abound, a method which obviously precludes 

 the possibility of any discrimination between the plant and 

 animal constituents. Since the waters are in constant 

 motion there is no strict necessity to pursue such food, but 

 an animal may obtain a sufficient supply, even while remain- 

 ing fixed, by causing a current to flow into its mouth or body 

 with the aid of cilia. This method of " current-feeding " 

 is employed by a great number of marine invertebrates from 

 sponges to sea-squirts. It can, of course, only be practised 

 in water, there is no equivalent for it on land. 



The food which is at the disposal of marine animals 

 is on the whole of a more indigestible nature than that 

 with which land animals have to deal. The floating plants — 

 diatoms — which form a large proportion of the plankton 

 have skeletons of silica, and the same substance occurs in a 

 number of marine animals also. Chitin, which forms the 

 exo-skeleton of Arthropods, is a substance second to none 

 in toughness. Lime is of particularly frequent occurrence, 

 either in the form of spicules (sponges, alcyonarian corals, 

 etc.), or of an external covering (shells and tubes). These 

 substances, not being internal like the vertebrate skeleton, 

 require to be first got rid of if many marine animals are to 

 get a meal — one of the advantages, or disadvantages, accord- 

 ing to the point of view, of an exo-skeleton ! In consequence 

 we are able to study among marine animals many interesting 

 arrangements for the preliminary treatment and mastication 

 of food, such as the vomerine and pharyngeal teeth of 

 fishes, the tentacles and horny beaks of cuttle-fishes, the 

 radulae of cuttle-fishes and Gasteropods, the pincers and 

 gastric mill of the higher crustaceans, the chitinous jaws 

 of Nereid worms, the protrusible stomach of starfishes, the 

 teeth and " lantern " of the sea-urchin, and so on. 



It is interesting to notice that the amount of food required 

 by marine animals for actual *' maintenance " is very low. 

 Most of the food consumed is utilised for growth and for 

 reproductive purposes, only a small fraction being used 



