Feeding and Digestion 179 



in actinians,''' and also in medusae."'* Blowfly larvae have active pro- 

 teases but weak carbohydrases in the gut, whereas the adults have good carbo- 

 hydrases and weak proteases. ''^ Carnivorous carabid beetles have good 

 protein digestion but weak and restricted amylase activity. '''" Fat digestion 

 is present in carnivores, but data are inadequate for comparison oF Fat digestion 

 in different groups oF animals. 



Merbi\'ores show weak proteolytic power but have active carbohydrate 

 digestion. Many marine and fresh-water molluscs (except cephalopods) feed 

 on phytoplankton or larger aquatic plants. In the pclecypods and the gastro- 

 pods possessing a crystalline style, the onlv extracellular digestion is of starch, 

 as in mussels, ''^ and clams and ovsters. '^*' Plant food is triturated in the 

 gizzard of Aplysia " ' and Limnaea. ■^- A style is present in all lamellibranchs, 

 although it is vestigial in the carnivorous septi branch bivalves. A style is absent 

 from carnivorous gastropods and is present in only those herbivorous gastro- 

 pods which feed by cilia or by slow continuous radular action. '^" 1 he tuni- 

 cate Ciona has practically no protein digestion but has active carbohydrases. 

 i!s.5 Nectar-feeding and leaf-eating insects may have several glandular sources 

 of amylase and maltase. Herbivorous mammals maintain in a rumen or cecum 

 a microflora which permits utilization of cellulose. 



In addition to the carni\'ores and herbivores, there are many extreme special- 

 ists. Most spectacular are the wood-eaters, which digest cellulose by their own 

 enzymes or by those of symbionts. The clothes moth is able to reduce the 

 disulfide linkages of keratin, making possible digestion of wool protein, and 

 the wax moth can utilize the polyglycerides of beeswax. A fly larva lives in oil 

 wells, digesting insects which fall into the oil, and possibly using symbiotic 

 bacteria which break down the hydrocarbons. '*'-" The lamellibranchs, a 

 predominantly herbivorous class, have made three attempts at further special- 

 ization. ^'"^ In the Teredinidae or ship-boring clams the valves are modified 

 for boring and a cellulase is present in the digestive diverticula. In the Tridac- 

 nidae of the tropical Indo-Pacific, phagocytic amoebocytes retain zooxanthellae 

 "which are farmed in the mantle edges." The Septibranchia, deep-water 

 molluscs, have the mechanical adaptations for meat eating in a crushing gizzard 

 but no extracellular protease. Other specialists rely on digestive enzymes 

 within their food. Leeches derive nourishment from a blood meal which slowly 

 autolyzes in the gut. "* Didiniwn uses a dipeptidase from Paranieciwn. Some 

 parasitic animals appear to have lost most of the essential digestive enzymes 

 and rely on their hosts; for example, trypanosomes '"' and parasitic Platyhel- 

 minthes, which have fewer enzymes than free-living flatworms. 



The animal groups most successful in digesting food are not the specialists, 

 howe\^r, but the omnivores with a full complement of enzymes, as some snails 

 (Helix), cephalopods, crustaceans, many insects, and the vertebrates. Proteins, 

 carbohydrates, and fats are all used by these animals. The various enzymes 

 may be mixed in one chamber, as in snails and crustaceans, or may be secreted 

 in successive parts of the alimentary apparatus, as in vertebrates. The tendency 

 toward diversification of enzymes appears even within a limited food habit. 

 Helix, for example, possesses a wider range of carbohydrases than any other 

 animal that has been examined; it has weak protein digestion, but is a very 

 successful genus. Fishes in general have a wide series of enzymes and some 

 are said to digest polysaccharides, such as lichenin and amygdalin, which may 



