340 APPENDIX. 



zoophytes and other minute marine animals; a crab, 

 from an inch to an inch and a half across, was frequent, 

 — and I observed the nest of one, formed by the small 

 branches woven together by a strong kind of thread, not 

 unlike that of which spiders make their webs ; it contained 

 a number of young ones." — Gardiner's Travels in 

 Brazils, ]p, 556, Dr. Johnston, in his Introduction to 

 Conchology, says, " Of the Gasteropods, some appear to 

 have been created expressly to dwell among the fields of 

 floating gulf- weed ; for the foot has been lengthened and 

 narrowed, and channelled down its middle, so that it 

 may receive the slender frond of the weed in the fur- 

 row, and give a firmer grasp and security to the crea- 

 ture. Of this beautiful adaptation the Scyllaea affords 

 a good example. The habits of the Litiopa are not less 

 worthy of notice. This is a small snail, born amid 

 the gulf- weed, where it is destined to pass the whole of 

 its life. The foot, though rather narrow and short, is of 

 the usual character, and, having no extra hold, the snail is 

 apt to be swept off its weed ; but the accident is provided 

 against, for the creature, like a spider, spins a thread of 

 the viscous fluid that exudes from the foot to check its 

 downward fall, and enable it to regain its pristine site. 

 But suppose the shock has severed their connection, or 

 that the Litiopa finds it necessary to remove, from a de- 

 ficiency of food, to a richer pasture, the thread is still 

 made available to recovery or removal. In its fall, 

 accidental or purposed, an air-bubble is emitted, prob- 

 ably from the bronchial cavity, which rises slowly 

 through the water, and as the snail has enveloped it 



