MR. A. HANCOCK ON THE ANATOMY OF DORIDOPSIS. 191 
into the tube. These are undoubtedly rudimentary salivary glands; and as their repre- 
sentatives in Doris open through the posterior wall of the buccal organ, it would appear 
that this dilated portion represents that organ, and the constricted part in front, which 
we have seen forms the inner wall of the exserted proboseis, corresponds to its narrow 
or anterior extremity. "Thus it would appear that this dilated portion, with the anterior 
constricted part of the alimentary tube, is really the buccal organ modified. 
The whole of the walls of this modified organ are muscular, the fibres, for the most 
part, being longitudinal and transverse. In the constricted portion, however, the walls 
are much thinner than in the dilated part, in which they are stout and firm. The inner sur- 
face of the whole (Pl. XVII. fig. 2 9, h) is divided into four longitudinal areas, but other- 
wise is quite smooth, the mucous membrane being entirely without wrinkles. There is 
no armature whatever, nor any process indicating a rudimentary condition of the tongue, 
so universal in the Nudibranchs,—the only exceptions being Tethys and Melibe, which 
are alike deficient in this respect: in these instances, however, the mouth is not trans- 
formed into a proboscis. 
The exsertion of this proboscidiform mouth through the oral aperture must be effected 
by the pressure of the fluid in the visceral chamber, aided, perhaps, by the muscular 
contractions of the walls of the sac or sheath. The sac itself will be everted much in 
the same manner as are the tentacles of a snail, the proboscis being attenuated and 
elongated by the contractions of the transverse muscles in its wall. The retraction of 
this organ, we have already seen, is provided for by the muscular bands passing from 
the posterior wall of the sac or sheath to the skin at the sides of the body. 
Besides the rudimentary salivary glands before mentioned, there are two large follicu- 
lated glands (Pl. XVII. figs. 1 & 2, j & i), which lie in front beneath the proboscis, and pour 
their secretion by a single slender duct into this organ immediately within the orifice. 
These glands are of a pale yellowish colour, and their ducts unite almost at once to form 
one common duct, which is at first considerably dilated, thus providing apparently a 
receptacle for the secretion; it then gradually tapers forward and, after a short course, 
sinks into the muscles of the foot, but may still be followed by dissection until it 
opens into the proboscis, just within the margin of the orifice, by a minute puncture 
(figs. 25 & 3 d). 
It is not easy to conjecture the nature of the fluid secreted by these glands. It can 
scarcely be salivary; for we have seen that the true salivary glands occupy the usual 
position, and, though minute, they may be deemed sufficient when we take into con- 
sideration the simple nature of the buccal apparatus, which is here reduced to a mere 
Suctorial tube, Can the secretion thus supplied be a solvent to enable the animal to 
penetrate the calcareous coverings of its prey, and so to reach its food ? Or may it be of 
à poisonous nature, to paralyze or benumb its victims? The feeble structure of the 
uecal organ seems to suggest the requirement of some such aid, as, in these animals, 
there is neither cutting nor prehensile organ of any kind. 
Immediately behind the true salivary glands the esophagus commences (Pls. XV.& 
lI. fig. 1). At this point of the alimentary tube we have seen that there is a constric- 
tion ; it at once again dilates and passes to the left, then turns to the right, and, crossing 
2D2 
