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, 
7% 2 
