XI PHYLUM MOLLUSCA 295 



cavity, the pericardium. It consists, in nearly all cases, of 

 only two chambers, an auricle and a ventricle. 



The nervous system and organs of special sense are in 

 most Gastropoda more highly developed than they are in 

 the fresh-water mussel. There are distinct cerebral and 

 pleural, as well as pedal and visceral, ganglia. Well- 

 developed eyes are present in the majority, and there are 

 otocysts, osphradia or water-testing organs, and usually 

 olfactory organs in the shape of special groups of cells on 

 the tentacles. 



The nephridia are granular tubes or chambers communi- 

 cating, as in the fresh-water mussel, with the pericardial 

 cavity on the one hand, and with the exterior on the other. 

 Two nephridia right and left may be present, or only one. 



The sexes are separate in some Gastropoda; in others, 

 such as the snails and slugs, they are united; and in the 

 latter case their structure is highly complex. The larva is 

 a Trochosphere, which subsequently develops into a form 

 known as the Veliger. In the Veliger the prototroch, or 

 ciliated prae-oral ridge of the Trophosphere, becomes drawn 

 out into a bilobed flap bordered with strong cilia. There 

 is a shell, a distinct foot bearing an operculum, and ten- 

 tacles and eyes are present on the head-region. The shell 

 is at first of simple conical form and the anus is placed in 

 the middle line posteriorly. It is only as development 

 advances that one side of the body becomes more rapidly 

 developed than the other, and the anus becomes shifted 

 forwards, the shell at the same time in the great majority 

 taking on a spiral form, and the visceral prominence 

 enclosed within it acquiring a corresponding shape. 



Two main divisions or sub-classes of the Gastropoda are 

 recognised — the Strep toneura and the Euthyneura. The 

 former comprises the majority of the marine univalves, 



