3322 THE MOLLUSKS OR SHELLFISH 



of which it passes through the alimentary canal. Professor Hutton says that it 

 will live for a week or ten days in fresh water, and more than a fortnight in salt 

 water, without being exposed to the air. The breathing orifice is situated on the 

 right side of the neck, and the radula shows some affinity to that of Physa, The 

 shell is solid, globular, with a short spire and an oval aperture. The animal is 

 furnished with a thin horny subspiral operculum. 



THE HIND-GILLED GROUP Order OPISTHOBRANCHIATA 



The Opisthobranchs form the second of the three main divisions of the gastro- 

 pods, and are all marine forms, having the sexes united in each individual, and 

 breathing chiefly by gills or branchiae. This characteristic at once separates them 

 from the Pulmonata, and the different positions of the branchiae, and their her- 

 maphrodite nature serve to distinguish them from the Prosobranchia, the third and 

 last main branch of the Gastropods. In the Opisthobranchs the branchial veins as 

 well as the auricle are placed behind the ventricle of the heart, but in the Proso- 

 branchs the branchial system is anterior to the heart. The majority of the mollusks 

 included in this order are unprovided with a shell in the adult state; but there are 

 some exceptions, such as the shell-bearing Pteropods, and many of the Tectibranchs. 

 They are generally furnished with a pair of tentacles and labial palpi, or an expan- 



CIRCUI.ATION IN Pleurobranchus aurantiacus. 



sion of the skin like the veil of the larval form. To comprehend the character 

 of the internal organization, the above illustration should be consulted. It repre- 

 sents a longitudinal section of the animal; p is the foot; #, the mouth, covered 

 above with the veil-like expansion, over which are the tentacles c; v shows the 

 branchial veins carrying the blood to the gills, from which it flows into the heart. 

 This position is the opposite of that which characterizes the Prosobranchs. An- 

 other anatomical peculiarity, which may here be referred to, is the direct communi- 

 cation of the system of blood vessels with the surrounding medium a characteristic 

 common to most other mollusks, and on which depends the changeable external ap- 

 pearance of the individual. In the illustration of Pleurobranchus, as above, g indi- 

 cates the opening of a duct which conveys water direct to the blood, and through 

 which the blood vessels permeating the back and foot, like the holes in a sponge, 



