194 INVERTEBRATE ZOOLOGY 



commissures may be either parallel (Fig. 92 A, orthoneurous) 

 or crossed (Fig. 92 C, streptoneurous) as a result of torsion 

 within the body of the animal. Alimentary canal, nephridia, 

 gills, circulatory and nervous systems are all affected in their 

 arrangement by this torsion, which is usually toward the right 

 side. The anus may open into the mantle cavity either on the 

 right side or in extreme instances may occupy a location near 

 the head. In this latter instance, gills, nephridia, and other 

 organs which belong primitively on the left side of the body 

 become shifted in position to the right, and vice versa. 



Digestive System. — The mouth may be a mere opening on 

 the ventral surface of the head or the opening at the end of a 

 tubular introvert or proboscis. The buccal cavity is provided 

 with an effective mechanism for breaking up food. This consists 

 of a rasplike radula whose teeth tear off particles of food. In 

 many instances, the radula is supplemented by a cutting or 

 crushing jaw of chitinous nature. The form, number, and size 

 of these radular teeth provide some of the most reliable criteria 

 for separating genera and species. The esophagus leads from 

 the mouth cavity to the stomach and this in turn into the 

 intestine, though the divisions of the digestive tract are usually 

 not sharply marked off in the convolutions within the visceral 

 mass. The anal opening is rarely at the posterior end of the 

 body. It is more commonly lateral or at the front end of the 

 body as the consequence of shifting of position of organs due to 

 torsion of the body. Various glands are associated with the diges- 

 tive system. Of these the liver which extends into the smaller 

 whorls of the shell and the salivary glands are the most important. 

 In some of the carnivorous snails, the salivary glands secrete 

 sulphuric acid. This acid aids the radula in perforating the 

 shells of other molluscs. 



Respiratory Organs. — Typically there are two gills contained 

 in the mantle cavity of the gastropods but this condition is 

 subject to various modifications. Very commonly as an accom- 

 paniment of torsion the primitively right gill degenerates, 

 leaving but one gill in most of the Streptoneura. In many 

 instances, true gills become replaced by other modifications for 

 respiration. Among the nudibranchs portions of the body 

 surface may become modified as gills. The true gills are vestigial 

 in the limpets and secondary branchiae are formed as a series of 

 folds between the mantle and foot. The most pronounced 



