SEA-FISHERIES LABORATORY. 189 
The siphons differ slightly in structure; the dorsal, or 
exhalent one (S?.d., figs. 2 and 3), 1s the shorter of the two. 
Its free edge is quite even and a small portion of the 
wall of the tip is exceedingly -thin. This thin tip is 
contractile, and is generally closed forming a little cone at 
the end of the siphon. The tentacles, which are rather 
over 1 mm. in length when fully extended, are situated in - 
an irregular ring at the base of this cone. Other and 
larger tentacles are borne on the wall behind this ring. 
At intervals the conical tip of the siphon opens and water 
and fecal matter are suddenly expelled. The ventral or 
inhalent siphon (Sv.v., figs. 2 and 3) remains permanently 
open. The free edge bears a great number of very delicate 
tentacles, smaller and thinner than those borne on the 
outer wall and differing slightly in structure. Behind 
this, as in the upper siphon, is a ring of tentacles with 
others scattered irregularly upon the wall. At the tip of 
both upper and lower siphons are a number of brown pig- 
ment spots which are the openings of little pigmented 
crypts or glands. 
_ The mantle edge (fig. 23) is thrown into longitudinal 
folds extending along its entire length. There is an inner 
strong fold projecting into the mantle cavity, a smaller 
median fold, and an outer fold which dips into the 
grooves on the shell margin. On the surface applied 
to the shell, the epidermis consists of rather irregular 
cubical cells, except towards the extreme edge, where the 
cells become spindle shaped and are crowded with brownish 
pigment granules. Over the rest of the mantle edge 
(ventral and internal surfaces), there is an epidermis 
consisting of a very regular layer of cubical cells with very 
- distinct nuclei and a strong cuticle. On the inner of the 
three folds mentioned above, and in the deep groove 
separating this from the small median fold, the cuticle 
