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—Philinidæ—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 Limnæus 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."[1]
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.
