Phylum Mollusca and Minor Eucoelomate Phyla 469 



rocky areas along the coast. When pried from their secure hold on 

 rocks, they roll up into firm l)alls ; mostly they are seen as flattened at- 

 tached animals browsing on the many algae which also dwell upon the 

 rocks. 



Underlying the eight dorsal plates is the mantle. It extends around 

 the margin and slightly over the ventral surface. Ventrally the mantle 

 is separated from the large muscular foot. 



The sexes are separate. The gonad is a large mid-dorsal organ 

 which opens through a duct and gonopore in the mantle cavity. The 

 fertilization of the eggs is external, and they may be discharged singly 

 or in strings. The zygote usually develops into a trochophore larva 

 which is free swimming for a short period before metamorphosing into 

 the typical adult. 



The Solenogastres are a peculiar group of this class which live 

 in deep water. They are wormlike in appearance and the mantle covers 

 the entire body. There is no shell, but tiny limy spicules are present, 



THE CLASS SCAPHOPODA — 



This small group of molluscs known as the tooth shells or tusk 

 shells (Fig. 156,m) live in sandy areas from low tide mark to great 

 depths. The curved tapering shell is open at both ends with the 

 larger opening at the head end. The foot, located at the head end, is 

 modified into a conical digging organ. There are no special sense or- 

 gans, but there are extensible filaments with suckerlike discs. These 

 possibly serve as tactile organs and also for grasping food. The mouth 

 contains a radula. There is a simple vascular system with a much- 

 reduced heart ; there are no gills — respiration takes place through the 

 mantle. Their symmetry is bilateral and is but little modified. The 

 sexes are separate and the fertilized eggs develop into trochophore 

 larvae. 



These unusual molluscs lie buried deeply in the sand with only 

 the open smaller end protruding into the water. Water is pumped in 

 and out of the mantle cavity through this opening. Food consists of 

 small organisms encountered in the sand. 



Shells of the genus Dentalium were often used by the West Coast 

 Indians as money. 



