210 xMESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OF DORIS. 
them at right angles. The transversely placed muscular plates above mentioned 
are most highly developed around the posterior lateral parts of the central cavity, 
and towards the front of the cavity show as a layer on each side, gradually de- 
creasing in thickness, and in front are continuous with each other, completing at 
that part the cavity containing the dentigerous membrane. This cavity is a laterally 
compressed tube, somewhat elongated from above downwards, and has projecting 
from the whole length of its posterior wall, almost as far as the front of the cavity, 
a fleshy lamina, in transverse section, wedge-shaped above, fusiform beloM\ This, 
which we shall call the cuneiform lamina, is not only connected with the posterior 
wall of the cavity, but also with the bottom of it, — the pouch at the lower and back 
part of the buccal mass ; its anterior border and upper end are both free. It divides 
the cavity into two nearly equal lateral parts. A further notice of it will find place 
in the description of the teeth and their development. 
If the upper portion of the above muscular conical mass be cut away, we find ex- 
posed on each side, a flattened dense substance* placed vertically, and being made 
up for the most part of indistinct transversely fibrous texture, and partially enclosed 
by the transverse muscular flakes before named. In front, the inner opposed surfaces 
only of these bodies or nuclei are clothed by the muscular flakes, which there form a 
thin layer, and their fibres are short, extending from the lower to the upper border of 
the nuclei ; further back the muscular coating is thicker and the fibres longer ; these 
being continued gradually more and more over the upper border of the nuclei, and 
then by degrees down the external surface, till at length they enclose the posterior 
end of the nucleus altogether, which is then free as it were in a muscular cavity, and 
the muscular flakes being then attached to the lower border only of the nucleus, but 
by one end to the inner and the other to the outer edge, play round the upper border 
somewhat like a belt over a pulley. 
By the action of this apparatus the tongue is not only partially everted from its 
tube, bringing numerous rows of spiny teeth to bear upon the food, but is retracted 
again into its previous position. 
Something similar to this perfect arrangement we have elsewhere described in 
Eolis, only there the lingual mass is single, whereas in Doris it is divided into two 
lateral halves. 
On the earliest approach of decomposition the dentigerous membrane-f- can be 
easily removed ; it is then seen as an incomplete tube, being quite open at the bot- 
tom, behind, and at the part corresponding to the cuneiform lamina, but retaining its 
original form. On looking into this tube, a fine membrane, continuous with the den- 
tigerous, is seen stretched transversely across ; and if the tube be replaced in its cavity, 
that membrane is found to fit upon, and clothe the top of, the cuneiform lamina. 
Thus we find that the tube of the tongue is divided into two unequal parts by this 
membranous septum, which occurs about the end of the upper or anterior third of 
the tube. The lower two-thirds of the tube thus constitute a large follicle, in which 
* Plate XIII. fig. 6. t Plate XIII. fig. 3, 
