Page:The Zoologist, 4th series, vol 4 (1900).djvu/335

Rh whence the mucus of the thread might presumably be derived. In many Pectinibranchs, however, in addition to the anterior gland, a ventral pedal pore exists in the median line of the anterior half of the foot-sole; it forms the opening of a cavity said to be comparable to the byssal-cavity of bivalves, and from it, externally, a well-marked groove often runs to the tip of the tail. It can hardly be doubted that the threads, in many Pectinibranchs, are derived from this ventral pore; in Cerithiopsis tubercularis, for instance, Jeffreys appears to have clearly seen the thread issue from "the opening in the centre of the foot-sole"; a narrow but deep groove extends from this opening to the tail, and Jeffreys tells us that it is by the tip of the tail that this animal attaches its thread to an object of support.

In Borneo, the writer was informed by Mr. Everett, certain land-operculates of this family, species of Alycæus, have the habit of suspending themselves, by a single thread, beneath overhanging ledges of the limestone rocks on which they abound. Mr. Everett often saw numbers hanging in this way, during rest, by threads which, to the best of his recollection, were sometimes an inch long; the habit, he adds, may "save them from the attacks of such foes as, for instance, Land-planarians, which are most frequently to be found in the same situations as these Mollusca, and which I have observed to prey on small Helices, which, however, have not the protecting operculum of the Alycæi." That molluscs thus escape certain enemies seems highly probable, and it is perhaps a mistake to suppose that operculates are already sufficiently protected; for Lucas, in Algeria, observed that numbers of Cyclostoma voltzianum, in spite of the operculum, are destroyed by the larvæ of a Drilus.

According to Swainson, "Megalomastoma suspensum, Guilding," is often found suspended by glutinous threads; and the remark is illustrated by a woodcut showing a shell of considerable size hanging from a twig by threads of moderate length, thirteen to fourteen in number, arranged upon the twig in four groups, but all proceeding from one point from between the operculum Zool. 4th ser. vol. IV., July, 1900.