A MANUAL OF AMERICAN LAND SHELLS. 15 



doucles and its whole body in various directions, as if seeking some 

 object on which to make a lodgment. The faculty of susjiending them- 

 selves in this manner indicates that they pass some part of their lives 

 on trees, from which they can thus make a convenient descent to the 

 earth ; there are some species, indeed, which are stated to inhabit trees 

 almost exclusively. It may serve also as a means by which they can 

 suddenly escape from the attacks of their enemies, and particularly of 

 birds. It is mostly, however, when they are young, or at least not 

 grown to their full size, that they enjoy this power. Those which have 

 attained their extreme dimensions and weight are too heavy to trust 

 themselves to so frail a support. They have no power to elevate them- 

 selves again, and in this respect are inferior to the spiders, which can 

 both lower and raise themselves by the aid of the secreted thread. 

 Like the si)iders, however, they often remain suspended in mid-air for 

 a time, and it is not unlikely that there is some pleasurable sensation 

 connected with the act, which induces them thus to i)rolong it. Dr. 

 Binney states that he had seen the descent actually practiced by 

 every one of our Atlantic species. 



Besides the watery fluid which at all times lubricates the integuments, 

 the animals can, at their will, secrete at any point, or over the whole 

 surface of their bodies, a more viscid and tenacious mucus than is 

 usually exuded. This i^ower is used as a means of defense. When- 

 ever a foreign substance touches them, immediately a quantity of this 

 mucus, of the consistence of milk and nearly of the same color, is poured 

 out and forms a kind of membrane interposed between themselves and 

 the irritating substance. So, also, when they are surrounded by a cor- 

 rosive gas, or are thrown into water or alcohol, they form over them- 

 selves in this way a thick protecting covering, which is undoubtedly a 

 non-conductor of heat and impervious, at least for a time, to liquids. 

 Shielded by this coating, they can live the greater part of a day im- 

 mersed in water, and for a shorter time in alcohol ; and M. Ferussac 

 asserts that they have survived for hours in boiling water. They leave 

 a trace of their usual secretion on every object over which they pass, 

 and thus can easily be traced to their retreats. The ordinary secretion 

 is most abundant at their posterior extremity. The secretion of the 

 mucous fluid over their surface is necessary to their existence. Death 

 immediately follows the failure of this power, and is preceded by the 

 drying up of the skin. 



