HABITS AND PROPERTIES. 5 



not grown to their full size, that they enjoy this power. 

 Those which have attained their extreme dimensions ;md 

 weight are too heavy to trust themselves to so frail a 

 support. They have no power to elevate themselves 

 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 spiders, however, they 

 often remain suspended in mid-air for a time, and it is not 

 unlikely that there is some pleasurable sensation con- 

 nected with the act, which induces them thus to prolong 

 it. We have seen the descent actually practised 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 power is used as a means of defence. Whenever 

 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 mem- 

 brane interposed between themselves and the irritating 

 substance. So, also, when they are surrounded by a 

 corrosive gas, or are thrown into water or alcohol, they 

 form over themselves in this way a thick protecting cov- 

 ering, 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 clay 

 immersed in water, and for a shorter time in alcohol ; and 

 M. Fe"russac asserts that they have survived for hours in 



