227 



find the number considerable. I shall only mention a few. 



The leech creeps up glass, etc., with great facility, attaching 

 itself merely by the viscid mucus its skin secretes. 



The common water newt or ascard will emerge from water, 

 and without assistance ascend the perpendicular sides of a glass 

 jar, or remain in contact with it for days together. These 

 creatures adhere only by their external secretion. 



The limpet adheres to the rock by the tenacity of its secre- 

 tion. Whenever it wishes to remove from its place it dimin.- 

 ishes the density of this by an injection of water amongst it. 



We have a still more familiar example in the ease with 

 which a snail will crawl over the smoothest surface. 



It has been asked, how can these various creatures ever 

 separate themselves if they attach themselves so closely by a 

 sort of fluid glue ? 



The answer is obvious — that the glue is never allowed time 

 to become solid, and that as long as it is fluid the insect can 

 as readily remove its feet from the glass as we can take off a 

 newly pasted paper from a wall. 



Since writing the above, my friend Mr. Byerley has drawn 

 my attention to the foot of the Acarus scarabcei, a parasite in- 

 festing the large black beetle in great numbers. 



In it the foot consists of a large circular disc, strengthened 

 diametrically by two strong processes or hooks, and divided 

 into compartments by a few lines which seem to be thickenings 

 of the membrane. 



These and the hooks are attached to a central substance at 

 the bottom of the last joint of the tarsus. 



This is acted on by two muscles which are able to retract it 

 powerfully within the joint. When in action the disc is fully 

 expanded (Fig. 40), and when the foot is raised, the two hooks 

 may be seen approximating until the whole assumes the ap- 

 pearance in Fig. 41. The membrane is probably dry, being 

 alwavs free from dust or dirt. 



