NUTRITION AND FEEDING MECHANISMS 247 



offal); tropic birds (fish and squid) (1, 18, 23, 26, 41, 55, 60, 75, 78, 88, 

 95, 96, 108, 109). 



Mammals. Carnivorous marine mammals include whales, seals, sea 

 otters and even bats. 



The toothed whales (Odontoceti) are hunters and exploit many forms 

 of nekton. The sperm whale Physeter feeds on fishes and especially cepha- 

 lopods. It is believed that squid are caught during deep dives. Porpoises 

 and dolphins are voracious feeders on small fish. The killer whales (Orca) 

 are powerful rapacious animals and are the only whales that attack other 

 cetaceans. They eat whole porpoises, seals, and kill walruses and large 

 whales. 



Seals differ greatly in feeding habits. Some seals are planktonic feeders. 

 Ringed seals (Phoca hispida) and harp seals (P. groenlandica) in the Canadian 

 Arctic eat mostly planktonic Crustacea, at least in summer. Crab-eating 

 seals (Lobodon carcinophagus) of the Antarctic are selective feeders, con- 

 suming krill. Fish plays an important part in the diet of eared seals 

 (Callorhinus ursina). Elephant seals (Mirounga leonind) of South Georgia 

 capture cephalopods; walruses (Odobenus) dive after bivalves. In antarctic 

 waters the large leopard seal, besides eating fish and cephalopods, attacks 

 penguins and other seals (25, 37, 72, 73). 



Among other carnivorous marine mammals we may note the interesting 

 sea-otter (Enhydra lutris), colonies of which occur off the west coast of 

 North America. Their food consists of hard-shelled invertebrates — clams, 

 sea-urchins, abalones, etc. — which they collect on the bottom. A curious 

 return to the sea has been made by certain peculiar piscivorous bats 

 (Noctilio, Pizonyx) which fly offshore and capture fish at the surface, 

 using their hind legs for that purpose (3). 



MECHANISMS FOR TAKING IN FLUIDS OR SOFT TISSUES 



These mechanisms are especially characteristic of parasites. 



Some parasitic polychaetes are described in Chapter 14. Ichthyotomus 

 sanguinarius is a blood-sucker, attacking eels (Myrus). It cuts into the 

 flesh of the fish by means of a pair of stylets and pumps in blood with its 

 pharynx. An anticoagulant is secreted by glands near the mouth (Fig. 

 14.9, p. 595). Marine leeches, such as Pontobdella which attacks rays, also 

 have piercing jaws and secrete an anticoagulant hirudin from salivary 

 glands. 



Certain gastropods, free-living and parasitic, fall into this category. 

 There are nudibranchs, such as Hermaea, which slit open algae (Codiurn, 

 etc.) with their radulae, and suck in the fluid contents of the cells. Dendro- 

 doris attacks compound ascidians, sucking in the soft tissues. Among 

 parasites may be mentioned ectoparasitic Aglossa with well-developed 

 suctorial proboscides (p. 596). There is also a semiparasitic lamellibranch 

 Entovalva, which lives in Synapta, and which apparently depends on fluid 

 matter for nutriment. 



Parasitic crustaceans often depend upon piercing and sucking to obtain 



