SEA-FISHERIES LABORATORY. 3 
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, 7’n. 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, mainly 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 
he, 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 eyes will be given in 
the chapter on those organs. 
When 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 
fibres of the adductor cross the body obliquely (figs. 46, 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 Mantie.—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 distinctly 
columnar towards the free edge of the mantle, and are in 
many places crowded with pigment granules of a dark 
