70 PHYSIOLOGY OF THE INVERTEBRATA. 



in a segmented, imbricated manner one behind the other. 

 The mouth is at one end of the body, and the anus at the 

 other. 



(4) The Hdcro'poda. — This order, which inchides Atlanta, 

 is sometimes immersed in that of the Gasteropoda. 



It may be remarked in passing that, although the Lamcl- 

 librancJiiata have no salivary glands, these organs are 

 frequently present in the Odontophora^ which include the 

 Scaphoiwda, Polyplacofliora, Gasteropoda, Pteropoda, and 

 Cephalopoda. 



(5) The Gasteropoda are subdivided into the Pidmogastcro- 

 poda and the Branchiogasteropoda. 



As an example of the digestive system of the Pulmo^ 

 gasteropoda we describe that of Helix, (the snail). 



The alimentary canal, which is much coiled, bends forward 

 to open by an anus in the mantle cavity. The mouth, 

 situated at the base of the head-lobe in front of the foot, is 

 bounded by lips. It leads into a buccal cavity, into which is 

 poured the secretion from two large salivary glands (Fig. 19, a)« 

 Then follows the oesophagus, which dilates into a crop or pro- 

 ventriculus. The crop leads into the stomach (provided 

 with a blind ca3cal appendage) passing into a long-coiled 

 intestine embedded in a large many-lobed organ — the so- 

 called liver, which opens by ducts into both the intestine and 

 stomach. The posterior portion of the intestine bends anteriorly 

 and widens into a rectum which opens, by an anal aperture 

 situated on the right side of the body, into the mantle or 

 pallial cavity. The intestine of Helw is folded internally so 

 as to form a typhlosole. 



The two salivary glands are on each side of the crop, but 

 their ducts open into the buccal cavity. These glands present 

 different degrees of development in different Gasteropods. 

 This is due to the construction of the mouth and the nature 

 of the food. In Heliv and Limax the salivary glands are 

 well-developed organs ; but in Calyptrwa they are simple 

 tubes. 



