20 PEOCEEDINGS OF THE 



marine animals, so that by carrying them on its back this crab 

 succeeds in a very remarkable manner in securing both self-con- 

 cealment and self-preservation. The two hind limbs o£ this crab, 

 then, being used for this particular purpose, cannot be engaged 

 for locomotion, so that, like the Hermit Crab, it is reduced to 

 a four-limbed condition, and I believe — although this is only a 

 matter of inference — that by its use of the chelae it is converted 

 practically into a hexapod, as in the case of the Hermit Crab. 

 The creature in question is very common in the Mediterranean, 

 and is I am told constantly to be seen in the Naples Aquarium, 

 but unfortunately that is a long way oif, and I have not as yet 

 had an opportunity of watching its movements. But the ex- 

 ample of Dromia which was met with on the Cornish coast was 

 peculiar in having adopted for its concealment a very unusual 

 device. It had not taken hold o£ a sponge or a Botryllus, 

 but a water-logged piece of cork, probably cork from a fishing- 

 net, and had either excavated it so as to fit its back, or the 

 piece of cork naturally did so ; and this piece of cork it was 

 holding firmly upon its back — a curious and unusual protective 

 covering. 



In order that my remarks may not be too monotonous and 

 too " crabbed " (if I may use the observation), I will pass on to 

 just a couple of instances which are noteworthy among the 

 Gasteropod Mollusca. 



Whilst staying at Plymouth, I was shown a certain mollusc 

 which had been found there in tolerable numbers, and upon 

 which Mr. Garstang (a friend connected with that laboratory) 

 had made some interesting observations. The mollusc in ques- 

 tion, a gasteropod, is one of those which walk upon the flattened 

 expanded ventral surface of the foot, and is known as the 

 Oscanius, or Pleurohranchus of some authors. It is about the 

 size of half-a-crown, and with much the same outline. It has a 

 very flattened body, notched in front and expanded. The actual 

 body of the creature is often thrown into folds, and its upper 

 isurface is more or less warty and coloured with blotches of 

 purple-brown. In front of its body on each side, and of course 

 above the foot, is a notch in the mantle-surface, in which is 

 a more or less concealed tentacle having a little eye-spot 

 close to its base. Now, on the right-hand side in the interval 

 between the mantle-border and its foot — when this mantle- 

 border is turned up so as to show it — we find concealed 

 underneath a curious cone-like ctenidium or gill, and in front 

 of this an organ, which is the external genital organ. Under 

 ordinary circumstances this Oscanius crawls about by wave- 

 like contractions of the foot, similar to those which may 

 be readily seen in the common Snail, commencing at the 

 posterior border and passing one after the other towards the 

 anterior border. As the result of these contractions, it glides in 

 a perfectly uniform smooth fashion over the seaweeds or rock 



