NUTRITION AND FEEDING MECHANISMS 241 



tions contain nerve poisons capable of paralysing prey, and a proteolytic 

 enzyme (p. 255) (100, 105). 



Chaetognaths are small voracious carnivores, chiefly planktonic in 

 habits. They are provided with a pair of chitinous hooks on either side of 

 the mouth, and with these they seize their prey, usually swallowing it 

 whole. Sagitta feeds on copepods, young fish and other arrow worms. 

 Spadella is a bottom-dwelling chaetognath which attaches itself to the 

 substratum and lunges at passing prey (85a). 



Echinoderms. Many ophiuroids are carnivorous, such as Ophiura and 

 Ophiocoma. Their prey consists of small polychaetes, molluscs and crusta- 

 ceans : these are captured by the arms and transferred to the mouth to be 

 swallowed whole. Asteroids show diversified carnivorous habits. Those 

 with pointed tube feet, such as Astropecten, live in sand and feed upon 

 small lamellibranchs. Others, with sucker tube feet, attack larger bivalves, 

 which they pull open and devour, e.g. Asterias, Pisaster. Snails, barnacles, 

 echinoids, even decapod Crustacea are attacked. Starfish have the remark- 

 able habit of everting their stomach over the prey if this be too large to be 

 swallowed whole, and digesting the prey before swallowing it (4, 38). 



Carnivorous Habits in Vertebrates 



Marine vertebrates are predominantly carnivorous and display much 

 variety in feeding habits, enabling them to exploit manifold sources of 

 food. 



Fishes. Cyclostomes are semiparasitic in habits. Lampreys fasten them- 

 selves to the bodies of other fish by means of a funnel-shaped sucker which 

 surrounds the mouth. Thus attached, they suck the blood and rasp off the 

 flesh of their prey with horny teeth which are borne on a piston-like 

 tongue. As the teeth wear away, they are replaced by new ones which 

 form underneath. Hagfishes are similarly armed with a powerful tongue and 

 lingual teeth, and soon reduce to a bag of skin and bones fish to which 

 they are attached. 



Among gnathostome fishes there is great diversity in food and feeding 

 habits. The kind of prey captured by a fish is dependent upon the structure 

 and habits of the fish, as well as the predatory species available. We 

 classify carnivorous fish into pelagic and benthic feeders, and note various 

 methods of locating and seizing prey in these two categories. 



Pelagic foragers hunt by sight, scent or touch. These animals are usually 

 provided with a well-developed strong dentition of pointed, cutting or 

 sometimes grinding teeth, which are renewed as they age or wear. In 

 sharks the older teeth in front of the jaw are shed and are replaced by 

 forward movement of more posterior teeth. In teleosts new teeth are 

 formed at the bases of the old, or in the spaces between. As examples of 

 active pelagic foragers which depend upon sight for hunting other fish, we 

 may mention mackerel, tunny, bluefish (Pomatomus) and barracudas 

 (Sphyraena). The jaws are armed with sharp teeth but are otherwise^un- 

 specialized, and agility and speed are used in pursuing the prey. Many 



