very ,few in number. The circular muscles of the ridges are 

 important in adjusting the positions of the ridges to each other 

 and thus in regulating the currents of water in respiration and 

 feeding, and in swimming. 



The sense organs of the mantle are of two distinct kinds, 

 tactile tentacles and eyes. The tentacles are distributed in two 

 bands along the margin of each lobe (figs. lo and 20). The 

 largest forms a broad band, several tentacles deep that runs 

 along the inner face of the margin of each lobe slightly removed 

 from the extreme edge, about where the base of the infolded 

 ridge joins the lobe. The tentacles in this ridge vary greatly 

 in size, those placed farthest from the margin usually being the 

 largest and those next to the free margins being smallest (fig. 

 26). Along the borders of the ears of the shell the tentacles 

 are somewhat longer and more slender than elsewhere, and they 

 are perhaps longest near the notch at the base of the anterior 

 ear. A large individual may possess several thousand ten- 

 tacles for there are from 75 to 100 on an inch of border. 



The other band runs along the face of the ridge near its free 

 border. In appearance these tentacles correspond to those of 

 the other band, but they are not nearly as numerous and are not 

 as large as the largest in the other band. They are most 

 abundant in the portion farthest removed from the hinge line 

 and are not found on the portions adjacent to the hinge. 



All of the tentacles of both bands are capable of being greatly 

 lengthened so they sometimes form a fringe along the border, 

 an inch or more in length. When the animal is disturbed they 

 are immediately withdrawn and form conical projections hardly 

 more than a sixteenth of an inch in length for the largest. 



The structure of the tentacles will be considered under the 

 head of sense organs. 



The eyes are placed along the margin of the larger band of 

 tentacles, on the side that is turned away from the free border 

 of the lobe of the mantle. They form a single scattered row in 

 which they are set at irregular intervals but fairly close together. 

 They are most abundant along the border farthest from the 

 hinge and are absent or very few in number for an interval near 

 the ears of the shell both anteriorly and posteriorly. In the 



IS 



