Mr. Alder on the Genus Polycera. 339 



Variety, white, beautifully marbled with dark reddish-brown and 

 orange. Two bands of dark chocolate-brown extend along each 

 side of the back, connecting the orange tubercles, and interrupted 

 by them. Branchial plumes speckled with brown and yellow. 



On first discovering this elegant variety I took it to be a di- 

 stinct species, especially as the tentacular filaments amounted 

 to six, while in the ordinary forms before examined they never 

 exceeded four. Further investigation, however, brought in- 

 termediate stages to my notice, and also taught me that the 

 number of tentacular filaments was extremely variable. The 

 veil, in fact, is surrounded by six prominent points, any num- 

 ber of which may be elongated into filaments, the rest re- 

 maining merely tubercles. Thus I have found individuals with 

 only one, two, or three of these elongated. Usually, however, 

 the four anterior ones become tentacular, the two lateral ones 

 very rarely so. The other appendages of these animals are 

 equally liable to variation. Specimens have occurred with 

 only one branchial lobe, and others without any, though no 

 appearance could be found indicating that they had been de- 

 prived of them by accident. 



The excellent description of Muller agrees perfectly with 

 this, excepting in the number of branchial plumes. These 

 have already been seen to be irregular, but I am inclined to 

 think the discrepancy to have arisen from his observing the 

 larger ones only and overlooking the smaller. No individual 

 coming under my observation has had fewer than seven 

 plumes. 



Several specimens of this pretty little mollusk have oc- 

 curred to me on small sea-weeds at low-water mark between 

 Tynemouth and Cullercoates, and also on the roots of Lami- 

 naria digitata thrown up by the tide. When kept in a glass 

 of sea water they are very active, but usually prefer swim- 

 ming at the top in an inverted position. I have never ob- 

 served them suspend themselves by threads, but they are very 

 fond of making a cup or sucker with the hinder part of the 

 foot, and suspending themselves by that means either from 

 the surface of the water, or by adhering to any foreign sub- 

 stance. The spawn I have found associated with them is in 

 the form of a short, broad riband, with a slight curvature, 

 and glued by one of its edges to sea-weeds. 



The pulsations of the heart observed at various times and 

 in different individuals, were from ninety to one hundred in 

 a minute*. 



* The question of the connexion between quick pulsation and rapidity 

 of movement in animals, is worthy of more careful investigation. I have 

 found some of the molluscous animals, whose motions are proverbially slow, 



z 2 



