SPINNING MOLLUSCS. 301 



surface of the water, stating that the point of attachment is 

 plainly visible, especially when the sun's rays fall upon the water 

 of the tank in which the animals are kept; he compares the 

 cupping of the surface-film (which can be conveniently examined 

 with a lens) to a small inverted parachute, and states that it is 

 shown to perfection when affected by the jerking motions of an 

 ascending Snail. The accompanying diagram, based upon the 

 observations here stated for Physa, will serve, in a general way, 

 to illustrate the use of threads by aquatic Pulmonates. (Fig. 3.) 



OPISTHOBRANCHIATA. 



Suborders Tectibranchiata and Nudibranchiata (Sea-Slugs). 

 Of Tectibranchs. only one family — Philinidce — is here represented ; 

 and that only by the following note by Mr. Spence Bate : — 



" The fact, observed by Mr. Warington, of the power of the Limnaus 

 to move from one place to another by means of a mucous suspending cord, I 

 have observed also to be the case with Bulla aperta [Philine aperta] in the 

 vivarium of my friend Mr. Smyth ; but the power of secreting the mucus, 

 which is exuded from the external surface of the animal, is limited in its 

 continuance; to prove the fact, we raised it three times to a glass shelf in 

 the vivarium ; the last time, not being able to secrete the ladder, it fell 

 head over heels, and therefore lost the power of choosing its place below, 

 as it could do when it came down by the cord." * 



In the Nudibranchs (typical Sea- Slugs) the production of a 

 thread has been noted in at least four families, and is perhaps 

 general. As the creatures are not ordinarily lighter than water, 

 they do not spin upward threads ; like Limax, like the Philine 

 just noted, and, like most other gastropods, they produce their 

 mucus-lines during descent. While crawling at the surface of 

 water, Alder and Hancock state: — " The Nudibranchs occasionally 

 drop suddenly down, suspending themselves from the surface by 

 a thread of mucus, which is fixed to the tail or posterior extremity 

 of the foot. In this way they will let themselves gradually down 

 to the bottom, or remain some time pendent in the water without 

 apparent support; for the thread of mucus is so transparent that 

 it can scarcely be seen. When carefully looked for, however, it 

 can always be perceived, originating in the track of mucus left on 

 the surface by the animal, the mucus forming a small inverted 



* Bate, 'Annals and Magazine of Natural History' (2), xv. (1855), p. 131. 



