CoTTinci.L. — J/iafoiiij/ of Siphouai-ia oliliquata. 083 



witli the rock, though not so closely as does that of the limpet. This is the 

 condition in which I have most freciuently found them. The thick body- 

 wall surrounding the internal organs on the sides (Plate XXVIIl, fig. 1 ; 

 hie) is composed of a dense mass of muscular tissue, the fibres of which run 

 in all directions, thus rendering a great variety of movement possible. 



In all regions of the body-wall forming the sides of the mollusc some 

 of the cells of the epidermis are differentiated to form multicellular mucous 

 glands, which project inwards among the muscles. The cavity of each 

 is club-shaped, the narrow part ending externally in a pore. Around this 

 cavity the cells are arranged — large glandular vacuolated cells, the out- 

 lines of which are not distinct. These cells secrete the mucus or material 

 which forms it. It is stored in the cells in the form of granules, which 

 staii; very deeply in borax carmine. These can be seen even under the 

 low power as conspicuous red masses. The secretion is discharged into 

 the cavity, and on the contraction of the muscles in the vicinity it reaches 

 the exterior. These glands are very numerous, and some of them are of 

 considerable size. When the animal is disturbed it shrinks up, causing 

 a considerable quantity of mucus to be exuded. 



Foot. Plate XXVIIl, figs. 2, 3. 



The foot is of considerable size, consisting of the whole of the ventral 

 surface behind the head. In the case of specimens preserved in formol 

 the foot is much shrunken (Plate XXVIIl, fig. 2 ; /) ; but I managed to 

 kill a fine specimen in a very well-distended condition, and then hardened it 

 in a strong spirit. This specimen shows very well the extent of the foot 

 (Plate XXVIIl, fig. .3 ; /) There is a slight propodium, on to which a large 

 pedal gland opens in the median line. This gland extends through the 

 whole length of the foot (Plate XXVIIl, fig. 3 ; pg, p). The foot, hke the 

 body-wall, is very muscular, and has the same structure as the latter, except 

 that here are no mucous glands, aiid in this region the epidermis is com- 

 posed of columnar cells. The beautiful movement of the muscular foot 

 may be well seen if a living specimen be made to creep up the inside of a 

 glass vessel. The action is similar to that of a snail. 



Head. Plate XXVIIl, figs. 1, 2, 3. 

 The head is marked ofE from the body by a slight furrow running trans- 

 versely across the ventral side immediately in front of the foot, and ex- 

 tending upwards for only a short distance on each side (Plate XXVIIl, figs. 1 

 and 2). Anteriorly the head is convex, and the thick wall investing it is 

 continuous with the bodv-wall described above, and has the same structure. 

 Ventrally the head is flat, and is in contact with the surface to which the 

 mollusc attaches itself. Like the foot, this part of the head is used for 

 effecting locomotion, and has the same histological structure. When dis- 

 tended the right and left lobes of the head can be well seen (Plate XXVIIl, 

 fig. 3 ; rl, II). No tentacles are present on the head, though part of these 

 lobes on either side may correspond to tentacles. 



External Orifices. Plate XXVIIl, figs. 1, 2, 3 ; Plate XXIX, figs. 2, 3. 



The mouth is situated in the middle of the ventral side of the head ; it 

 is a transverse slit, somewhat crescentic in outline, and median in position 

 (Plate XXVIIl, fig. 3 ; m). 



On the right side of the body is the respiratory orifice, situated half- 

 way along that side (Plate XXVIIl, fig. 1 ; r). In large specimens it is about 



