320 THE ZOOLOGIST. 



specimens in a basin of sea-water, observed that they crawled to 

 the edge and suspended themselves by a thread.* 



Explanation of Plate III. 



Fig. 1. — A Water-Snail (Limncea auricularia) hanging by a thread from 

 the surface of the water in an aquarium. After Taylor, ' Monograph of the 

 Land and Freshwater Mollusca of the British Isles,' i. (1899), p. 318 (fig. 610). 



Fig. 2. — A fresh-water Limpet (Ancylus lacustris) using a thread. From 

 a sketch by Prof. Cockerell. 



Fig. 3. — Diagram illustrating the use of threads by aquatic Pulmonate 

 Molluscs, based for the most part on observations recorded for Physa hyp- 

 norum, by Mr. G. S. Tye. The animal is ordinarily slightly lighter than 

 water: — a, an individual crawling through the water towards the surface, 

 leaving its locomotory mucus behind in the form of a thread, which retains 

 the animal, and prevents its sudden rise to the surface ; b, the animal at the 

 surface, taking in a supply of air — the thread, having been continued as a 

 floating slime-trail, is now attached to the surface ; c, the animal returning 

 by descending its thread ; another individual is making use of the fixed 

 thread for ascent. An upward journey (a) may be abandoned, the animal in 

 that case returning upon its unattached thread, d. 



Fig. 4. — " Megalomastoma suspensum," a land operculate of doubtful 

 identity, at rest, suspended by a number of threads from a twig ; probably 

 much enlarged. After Swainson, ' Treatise on Malacology,' 1840, fig. 29 ; 

 presumably from a sketch by Guilding. 



Fig. 5. — Chondropoma dentation, a land operculate, at rest, suspended by 

 a short thread ; slightly enlarged. After Binney, ' Terrestrial Air-breathing 

 Mollusks of the United States,' ii. (1851), p. 347. 



Fig. 6. — Cerithidea obtusa, a brackish-water, somewhat amphibious, 

 operculate mollusc, at rest, suspended to a bough by a number of short 

 threads. After A. Adams, ' Zoology of the Voyage of H.M.S. Samarang : 

 Mollusca,' 1848, pi. xiii. fig. 3 b. 



* Dennis, in Jeffreys, torn. cit. p. 386. 



