NUTRITION AND FEEDING MECHANISMS 233 



collect sand and food particles, the small circumoral spines pushing the 

 food into the gut. Somewhat similar habits are displayed by burrowing 

 ophiuroids (Amphiura, Ophiopsila, etc.), which move particles and detritus 

 along the arms to the mouth by means of tube feet. 



Crustacea. Many littoral amphipods feed on organic detritus, largely 

 vegetable matter. Corophium, a burrowing form, sometimes filters off fine 

 particulate matter, but to a much greater extent feeds by selecting particles 

 from the mud in which it lives. When behaving as a selective deposit 

 feeder, Corophium scoops up and sifts small quantities of mud with the 

 gnathopods. Larger particles are conveyed to the mandibles, where they 

 are crushed by molar processes and swallowed. Smaller particles are 

 retained by a fringe of setae on the gnathopods, sifted and then transferred 

 to the mandibles to be swallowed. 



The Cumacea are small burrowing animals which employ filter-feeding 

 (p. 228) or feed on small micro-organisms occurring in the soil detritus. 

 Cumopsis collects food by cleaning off sand grains and other small objects. 

 These are picked up by the first pereiopods and manipulated and cleaned 

 by the maxillipeds. The food is then passed to the mouth parts (24). 



Scraping and Boring 



Here are included devices which enable animals to bore into hard 

 materials, the fragments of which are swallowed and digested; or to 

 scrape off encrusting material and organisms; or to rasp and bore into 

 living prey or dead animals. Invertebrates which feed in this manner 

 include various echinoderms, molluscs and Crustacea. Certain fishes are 

 also included in this category. 



Echinoderms. Sea urchins possess a set of strong teeth forming a biting 

 and scraping apparatus known as Aristotle's lantern. With this structure, 

 rock-dwelling forms such as Echinus are able to scrape off and masticate 

 encrusting organisms; bottom material is also conveyed to the mouth 

 by tube feet. 



Molluscs. Especially suitable for scraping is the radula of chitons and 

 gastropods. This is a horny ribbon covered with many rows of small 

 recurved teeth (Fig. 5.25). The radula lies on the ventral side of the buccal 

 cavity and frequently works in conjunction with the palatal plate or jaws. 

 Growth of the radula is continuous during the life of the animal and takes 

 place in a ventral diverticulum known as a radular sac, in which proliferat- 

 ing tissue gives rise to transverse rows of cells (odontoblasts), forming 

 new teeth, and other cells forming the horny base of the ribbon. As a 

 result of posterior growth the radula is pressed forwards and a new surface 

 constantly replaces that worn away. 



The radula is supported by cartilaginous masses providing attachment 

 for protractor and retractor muscles by which the odontophore apparatus 

 is protruded from, or withdrawn into, the buccal cavity. Rasping movements 

 of the radula are brought about by action of another set of protractor 

 and retractor muscles, by which it is drawn backwards and forwards over 



M.A. — 8* 



