DIGESTION 255 



INGESTION, STORAGE AND TRITURATION OF FOOD 

 Reception 



The region of reception includes the mouth, buccal cavity and pharynx, 

 together with those diverse ancillary structures employed in feeding — 

 for example, ciliated fields, biting mouth parts, radulae, jaws and sucking 

 apparatuses. The work of these structures in gathering or seizing food has 

 already been described (Chapter 5). Digestion of foodstuffs takes two forms : 

 the food particles are taken in by cells and broken down intracellularly ; 

 or they are attacked by digestive enzymes in the gut cavity and the soluble 

 products are absorbed. Food material has to be selected of a size that can 

 be swallowed ; or, if of excessive size for the gape, it must be reduced to 

 suitable dimensions. The food of filter and detritus feeders is preselected 

 and consists of fine particles. Scrapers and borers, by means of mechanical 

 aids, obtain their food in particulate condition. Carnivores and omnivores 

 break down prey or food masses by tearing them apart with appendages or 

 mouth parts ; grinding them up in the mouth or gizzard ; or by subjecting 

 them to chemical action. In any event the food has to be rendered particul- 

 ate in order to permit phagocytosis, or to provide maximal surface for 

 enzymatic action. 



In the anterior gut region there are teeth in the buccal cavity of verte- 

 brates; radulae and jaws in chitons, gastropods and cephalopods; and jaws 

 on the eversible pharynx of certain errant and carnivorous polychaetes. 

 The muscular pharynx of the polychaete Aphrodite serves as a gizzard for 

 crushing the food. 



In some animals preliminary chemical action is used to attack and break 

 down large food masses preparatory to swallowing. Certain carnivorous 

 gastropods which lack a gizzard — e.g. Dolium, Cassis— secrete free acid 

 from the buccal glands and use it for dissolving calcareous matter in their 

 food (consisting of other molluscs and echinoderms). A poisonous secre- 

 tion is produced by the buccal glands of certain gastropods (Toxiglossa) 

 and by the posterior salivary gland of the octopus. In the latter animal 

 the gland produces amines having powerful effects on the nervous system, 

 such as tyramine, octopamine and hydroxytryptamine. These toxic 

 secretions are used to immobilize the prey (5a, 27). Equally specialized are 

 the salivary glands of blood-sucking ectoparasites such as leeches and the 

 polychaete Ichthyotomus, which produce an anticoagulant (p. 595). The 

 secretion of the salivary or sublingual glands of lampreys also prevents 

 coagulation of the blood of fishes on which the lamprey feeds. 



Proteolytic secretions are poured over the food in some instances to 

 reduce it to semi-liquid form, a process termed extra-intestinal digestion. 

 To cope with large food masses starfish evert the stomach and pour 

 proteases over the food, and Portuguese men-of-war discharge ferments 

 through the gasterozooids which adhere to the prey. A polyclad Leptoplana 

 initiates digestion outside the body by exuding protease through the everted 

 pharynx over the food mass. The enzyme in these instances is secreted by 



