NUTRITION AND METHODS OF FEEDING 191 



is exerted which, though not immediately effective, will, 

 if maintained for a period of some minutes, cause the shell 

 to gape. Immediately this occurs, the starfish is able to 

 insert the protrusible margins of its stomach into the shell 

 cavity and to dissolve out the contents. The efficacy of the 

 method depends, of course, upon the fact that keeping the 

 shell-valves closed implies an active or contracted state of 

 the animal's muscles which cannot be prolonged indefinitely. 

 Experimentally, it has been demonstrated that the tube- 

 feet of a starfish will maintain a pull on the shell equivalent 

 to 1350 grams ; this strength, when applied for a period of 

 about fifteen minutes, is amply sufficient to open a Venus 

 (Schiemenz, 1895-7, and cf. p. no). Starfishes are not 

 entirely dependent upon bivalves for their living, but con- 

 sume in addition a quantity of " lesser fry " wafted to 

 them by the cihary currents set up by the cells of the 

 epidermis. 



Sea-urchins may browse on seaweeds or even eat rock- 

 barnacles (Roaf, 1 9 10). When a sea-urchin is feeding, the 

 pedicellarise, spines, and tube-feet all participate in conveying 

 food to the mouth. In sand-dwelling species such as 

 Echinocardium and Spatangm the buccal tube-feet are said 

 to be extended and to broaden out terminally in order to 

 seize particles and pass them first to the spines of the lower 

 lip and thence, with the assistance of the upper lip spines, 

 to the mouth. There is no shovelling of sand into the 

 mouth while the animal is ploughing through it (Hornyold, 

 1909). Sea-cucumbers feed by plunging their tentacles, 

 which are sticky, into the sand, and transferring the adherent 

 particles to the mouth. 



Nemerteans, or ribbon-worms, are usually stated to be 

 entirely carnivorous. In addition to capturing living 

 animals of small size with the aid of their proboscis, they 

 probably do a good deal of scavenging and will almost 

 certainly consume a quantity of decaying vegetable matter 

 as well. The form Lineus mariniis has been observed, in 

 the aquarium, to attack and devour the tube- worms Sabella 

 pavonina and Serpula (Curator's Rept. Port Erin, 1906). 



