256 THE BIOLOGY OF MARINE ANIMALS 



the stomach or coelenteron. The octopus predigests its prey by discharging 

 a protease into it. This is said to arise in the posterior salivary gland, along 

 with toxic substances (27, 36a). 



Salivary glands are frequently present in the anterior gut region. The 

 glands are given topographical names according to their location, e.g. 

 buccal glands opening into the buccal cavity, sublingual glands discharging 

 under the tongue or radula and pharyngeal glands opening into the 

 pharynx. The primitive function of these glands is that of secreting mucus 

 for lubricating the food — for example in triclad turbellarians, gastropods 

 such as Patella, and cephalopods. Unicellular mucus glands are of general 

 occurrence in the gut epithelium as well. In certain animals which have 

 acquired extracellular digestion, the salivary glands secrete digestive 

 ferments. The kinds of enzymes produced by the salivary glands differ 

 according to the animal's diet. Thus, in many carnivorous gastropods the 

 secretion effects a preliminary digestion of protein, and in herbivorous 

 opisthobranchs it attacks carbohydrates. Secretion of special substances 

 for use outside the body has been mentioned {vide supra). 



Conduction and Storage 



Food is conducted to the digestive chambers by the oesophagus. In 

 certain animals the oesophagus is dilated into a crop for storage purposes. 

 In leeches, for example, which depend upon large meals of blood at in- 

 frequent intervals, the crop forms a large part of the gut. A crop is present 

 in herbivorous gastropods such as Patella, Aplysia and Haliotis. The large 

 gastric cavity in fishes may be considered a storage as well as a digestive 

 chamber; when a stomach is wanting, the anterior intestinal region is 

 similarly enlarged (e.g. Chimaera, Fundulus). Some preliminary digestion 

 often takes place in the crop, by enzymes regurgitated from more posterior 

 regions (herbivorous gastropods). 



Trituration in Gizzards 



Following the oesophagus, the anterior region of the gut is frequently 

 specialized as a grinding organ or gizzard for reducing food to particles 

 small enough to be further manipulated by the digestive apparatus. A 

 gizzard lined with chitinous teeth is present in various opisthobranchs 

 (e.g. Aplysia) and pteropods. It is particularly well developed in Scaph- 

 ander (Bullidae), where it contains several tough plates capable of crushing 

 shells which are swallowed whole. Septibranchs (Lamellibranchia) are 

 scavengers, and possess a powerful crushing gizzard capable of breaking 

 up large food masses into particles small enough to be ingested by cells 

 of the digestive diverticula. The pyloric stomach of certain bottom-feeding 

 fishes which ingest sand and mud is bulbous and highly muscular (e.g. 

 Mulloides). Trituration is aided by sand taken in with the food (1, 24, 

 31,35,49). 



The stomach of Crustacea is a capacious organ, ectodermal in origin 

 and lined with chitin. Here the food is broken up and mixed with digestive 



