SPINNING MOLLUSCS. 305 



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 Pecti- 

 nibranchs, 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. 



Cyclophorid-e. 



In Borneo, the writer was informed by Mr. Everett, certain 

 land-operculates of this family, species of Alycceus, have the habit 

 of suspending themselves, by a single thread, beneath over- 

 hanging 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 

 Alycai" 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 larvae of a Drilus.* 



According to Swainson, " Megalomastoma suspension, Guild - 

 ing," 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 



• * Petit de la Saussaye (quoting Lucas), ' Journal de Conchyliologie ' iii. 

 (1852), p. 100. 



Zool. 4th ser. vol. IV. , July, 1900. y 



