GASTROPODA. 



371 



marginal tooth alone is present, and that central and laterals are absent alto- 

 gether ; in the Rachiglossa (Fig. 293 C] it is 1.1.1., which means that a central 

 and one lateral on each side are present ; the teeth are strongly cusped in this 

 group. In the Taenioglossa (ribbon-tongned) the typical formula is 2.1.1.1.2. 

 (Fig. 293 B). In the Ptenoglossa there are an indefinite number'oJP outer teeth, 

 not distinguishable into marginals and laterals, and the formula is written 



oo. 1. oo. The Rhipidoglossa (fan-tongued) are characterized by an indefinite 

 number of marginals, arranged like the ribs of a fan; typical formula 



00.5.1.5. oo. (Fig. 293 A}. The Docoglossa have a few, but strong columnar 

 teeth, and the special feature is the multiplication of the central ; thus Patella 

 has 3.1.4.1.3., or, as it may be written, 3.1. (2 + + 2) 1.3. 



In the Euthyneura there is generally no distinction into marginals and 

 laterals. The length of the radula, and the number of rows of teeth, varies 

 much in different species. In Patella it is very long, in the Pulmonata it is 

 short and broad. The radula is entirely absent in the Eulimidae, Pyrami- 

 dellidae, Thyca, Entoconcha, Entocolax, Coral- 

 liophilidae, certain species of Terebra, Torna- 

 tinidae, Cymbuliopsis, Gleba, Doridiidae, 

 Doridopsis, Corambe, Phyllidia, Tethyidae 

 (i.e., in parasitic and suctorial forms). 



The subradular organ is a papilla in front 

 of the radula on the floor of the mouth. 



In addition to buccal glands, some- 

 times found round the buccal opening, 

 there is always a pair of salivary 

 glands opening into the buccal-cavity. 

 The buccal-cavity leads into the oeso- 

 phagus, which is followed by a dilated 

 stomach, usually provided with a caecal 

 appendage. The stomach opens into 

 an intestine, which is usually long and 

 coiled, and surrounded by a large multi- 

 lobed liver. The liver occupies nearly 

 all the upper coils of the visceral sac, 

 and pours its secretion into the intes- 

 tine and into the stomach (Fig. 295). 



FIG. 294. Alimentary canal of Aeolis 

 papillosa (after Hancock). Bm 

 buccal mass ; Oe oesophagus ; M 

 stomach ; L liver caeca which enter 

 the dorsal appendages (cerata) : A 

 anus. 



The arrangement of the alimentary canal and of the liver presents in detail 

 many essential modifications ; one of the most remarkable being that offered by 

 the stomach with its diffuse hepatic caeca (often extending into the cerata) in so 

 many of the Nudibranchiata (Fig. 294). The terminal part of the intestine is 

 distinguished by its size, and may be called the rectum. 



The oesophagus may have a crop-like swelling in its course, and sometimes 

 there is a gland on about the middle of its length (gland of Leiblein, Fig. 

 295). The stomach may contain in its middle region masticatory plates 

 (some Opisthobranchs), or be divided by constrictions into three regions 

 (Aplysia). 



