2 2 PROCEEBIIf GS OF THE 



a large lobe whicli extends behind and bears upon its upper 

 surface posteriorly tbat little borny secretion which furnishes 

 a sort of lid for closing the mouth of the shell when the animal 

 has retracted itself under its protection. Now this creature 

 {Xenopliora) crawls about invariably upon what is called a rubbly 

 bottom, that is to say, upon the bottom of the sea, where the 

 surface is mainly composed of fragments of coral, bits of stone, 

 and objects of that description. JSTow if this Xenopliora had a 

 smooth sucking-foot upon which it had to glide, as in the case 

 of Oscanius, it would be very badly off" indeed : it would 

 have to go up and down, as it were, these enormous boulders : 

 and it would be in much the same condition as a man would be 

 who had to make progress over boulders which were perhaps 

 three or four times his own lieight. His progress would be 

 a very slow and altogether a very tedious business. But this 

 Xenophora gets over the trouble very easily indeed, because 

 its foot, Avhich is fairly long and narrow, is simply extended 

 until one end touches a given stone, the other end holding on 

 to the pebble or stone behind, and when it has got a good hold 

 in front, it lets go its hold behind, and draws the posterior end 

 towards the anterior and fixed end. So that the foot is formed 

 into a sort of loop ; and thus by a series of steps, as it were, it 

 strides from rock to rock, instead of going up and down in the 

 intervals between them. Now whilst this procedure is going on 

 as its normal locomotive business, we find other curious features, 

 especially when it is not crawling, but when it is resting. We 

 find that this mantle-lobe, which is protruded from the mouth of 

 the shell for the purpose of increasing the size of the shell by 

 secreting fresh material to its margin, is also doing something 

 else. This mantle-lobe picks uj) in the migration of the creature 

 bits of stone, shell, or coral, and holds them quite firmly. It 

 holds these fragments there quite steadily until, by secreting the 

 proper shelly substance on the under surface of the piece of stone 

 or shell, it cements this to the outer surface of its own shell, Avhich 

 also it is building up by secretion ; thus, by the continuance 

 of this process, we find that the animal conceals itself in the well- 

 known fashion by means of masses of stone and shell cemented 

 to its abode — entirely concealing its form and its identity, 

 so that, when resting, no one would recognize this as a living 

 creature, and would simply imagine that it was a little aggre- 

 gation of shells or rubble, such as might be seen present all 

 round it. 



Here we have in these two forms, both belonging to the common 

 group of Gasteropods,two extreme modifications of foot-structure. 

 Of course there are other forms equally remarkable amongst 

 the Mollusca, and many especial modifications for locomotive 

 purposes ; but these two I have mentioued simply because 

 one was obtained where I was investigating the movements of 

 crabs and other creatures. Of course there were many other 

 forms which came under observation at the same time ; but in 



