21 



until the dorsal margin is reached. The tentacles (fig. 1, 

 Tn.) are long, very extensible and active on the outer fold, 

 while those arising from the velum (fig. 1, Tn. v.) are short 

 and move but little. When fixed they appear papillose, 

 but this is due probably to the great difficulty in fixing 

 them without contraction and folding of the surface tissue. 



The outer tentacles are roughly separable into two 

 groups, a series of short tentacles, mainl}' one row deep, 

 lying next to the shell, and longer ones capable of much 

 extension and contraction inserted in one or two irregular 

 rows. The former are unpigmented in both valves, and 

 lie, when the shell is opened, curved back over the shell. 

 The others of the upper or left valve have a streak of 

 pigment on their upper sides, and a similar, but less 

 intense, streak is present to the same side of these 

 tentacles on the lower valve. 



Further details in regard to the ej^es will be given in 

 the chapter on those organs. 



AVhen the valves of the shell are separated the two 

 vela hang at right angles to the plane of the valves, just 

 touching, like two curtains. The small tentacles lie 

 across one another, and form a rude grating. The velum, 

 as we have seen above, is of great importance in connec- 

 tion with locomotion. It has been pointed out in 

 considering the muscle impressions on the shell that the 

 hbres of the adductor cross the body obliquely (figs. 40, 47, 

 A. s.), the result is that the right mantle lobe has a free 

 portion of much greater area than the left. 



Histological Structure of the MA^■TLE. — Over the 

 whole surface of the mantle there is a single layer of 

 cubical or columnar epithelial cells, forming the 

 epidermis. These cells become much more distinctlj^ 

 columnar towards the free edge of the mantle, and are in 

 man}' places crowded with pigment granules of a dark 



