304 THE ZOOLOGIST. 



PECTINIBEANCHIATA. 



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 observa- 

 tions ; 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 pur- 

 poses 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 gastro- 

 pods. In some cases figures of the foot show conspicuously the 

 long transverse slit-like opening of an anterior pedal gland, 



