NUTRITION AND FEEDING MECHANISMS 239 



Autolytus edwardsi, attack hydroids. They cut off the tentacles or pene- 

 trate the coelenteron by means of pharyngeal teeth, and suck in hydroid 

 tissue and fluids through a protrusible proboscis (34, 83). 



Crustacea. Excluding the filter-feeders these animals are generally 

 omnivorous, although many species are chiefly dependent on living prey 

 or carrion. Food material is grasped by head or thoracic appendages and 

 masticated by the mouth parts before being swallowed. Some species 

 capture large particles and prey to supplement filter-feeding. Mysids, for 

 example, seize small animals (crustaceans, arrow worms) with thoracic 

 limbs and tear them up by means of mandibles and maxillules. Isopods 

 are frequently carnivorous or scavenging in habit. Chin'dotea, for example, 

 seizes carrion with its gnathopods and bites off pieces with the mandibles 

 (98). 



Chelae and chelipeds are used by decapods for seizing, manipulating 

 and shredding food. In the prawn Palaemon the chelipeds convey pieces of 

 food to the maxillipeds, which hold them while fragments are torn off by 

 the mandibles and other mouth parts. In lobsters (Nephrops, Palinurus) 

 food is held by the mandibles, while it is torn up by the action of the third 

 maxillipeds prior to being swallowed. Similarly in the shore crab Carcinus 

 maenas the food is shredded and torn before it is swallowed. Pieces seized 

 by the chelae are transferred to the mandibles, which hold them while 

 they are being torn into fragments by the other mouth parts. Algae have 

 little or no food value for larval prawns (Palaemonetes), which need 

 animal food for survival (10a). 



Molluscs. Many gastropods are carnivorous in habits, feeding on living 

 prey or carrion. Some species swallow their prey whole. Tectibranchs, 

 such as Scaphander and Bulla, swallow entire lamellibranchs, which they 

 grind up in a muscular gizzard. Special predatory habits are also en- 

 countered among nudibranchs. Ca/ma, for example, feeds on the eggs 

 of shore fishes, which are slit open with the radula and the egg contents 

 extracted. Other gastropods masticate the food to some extent before 

 swallowing it. Thus Pleurobranchus grasps pieces of carrion with its mus- 

 cular proboscis and rasps off bits by means of the radula. It is likely that 

 many of the carnivorous gastropods secrete protease from salivary glands 

 and this assists the radula in breaking up the food. Gymnosomatous 

 pteropods are also carnivorous in habit and feed largely on thecosomes. 

 These animals possess an eversible proboscis provided with various devices 

 for seizing prey, namely hooks, suckers and sticky secretions. Supplement- 

 ing these devices are jaws and powerful radulae (70). 



Among bivalves one group, the septibranchs, have become carnivorous 

 in habits. Cuspidaria and Poromya are burrowing forms which keep the 

 siphonal openings at the surface. In this position they draw in small 

 animals, living or dead, which chance to be in the vicinity, through the 

 large inhalant siphon. This is accomplished by aspiratory movements of a 

 transverse muscular septum which replaces the branchiae of other lamelli- 

 branchs, and which divides the mantle cavity into upper and lower chambers 



