PECTINIBRANCHIATA.
In Pectinibranchiata the spinners occur chiefly among the mainly phytophagous kinds, which live on seaweed growing near the shore or floating on the surface of the ocean, on aquatic plants in brackish and fresh water, and on branches and aerial roots of trees, bushes, &c, by the water's edge and on land; we have also a few notes for kinds living on rocks, coral, &c.; as well as for others whose habitats afford less facilities for the exercise of the faculty. Among the notoriously carnivorous Sea-Snails, Whelks, Murices, Purples, &c, we have no observations; nor have we any for the Volutes, Olives, Harps, Cones, &c.; among the Mitres, we have one known spinner; and among the Cowries, one. Some of these animals are of large size, but nearly all those with which we are concerned are small, or of moderate growth. These creatures, like other spinning molluscs (except air-breathing Water-Snails), are generally heavier than the medium in which they live, and thus they spin during descent; the threads in many cases are doubtless used for purposes of locomotion; in other cases, however, their chief function seems to be the retention of animals liable to be shaken from their supports during high waves and winds; in still other cases the threads appear to serve chiefly as means of attachment and suspension during repose, the creatures being upheld at this time sometimes by one and sometimes by several or many threads. Most spinning Pectinibranchs, no doubt, are able to ascend to their former positions by crawling up the suspensory thread; this has been observed in Litiopa, in Valvata, and perhaps in Rissoa.
In the molluscs above considered—air-breathing Water-Snails and Sea-Slugs—the threads are doubtless of the nature of those of Limax, being derived from anterior glands, and representing the mucus-trail of ordinary locomotion. The same is probably the case with some of the spinners of the present order; but the writer is doubtful on this point, for the foot in Pectinibranchs, often of peculiar construction, serves for locomotion, differing somewhat from that with which we are familiar in other gastropods. In some cases figures of the foot show conspicuously the long transverse slit-like opening of an anterior pedal gland,
