342 Mr. Alder on the Genus Polycera. 
typical form might perhaps be considered sufficient to war- 
rant the establishment of a new genus. Mr. Forbes has stated 
that the absence or presence of eyes is generic in this group. 
I am by no means confident of this. In groups where these 
organs are fully developed, the function performed by them is 
of sufficient importance to give them a primary character ; 
but where these or any other organs are reduced to their mi- 
nimum of development, so as, in fact, to become merely rudi- 
mentary, their absence then becomes of little importance. In 
the present case we are unable to prove that these minute 
black spots perform any of the purposes of vision: why then 
may they not be mere indications of organs which are to re- 
ceive a further development in other forms of the same group? 
Such rudimentary organs, incapable of performing any real 
function, are not unknown in other departments of zoology, 
and form a beautiful illustration of the very minute gradations 
through which the development of organs is carried in the 
whole range of animated nature. 
The number of parts in this class of animals is not to be 
depended upon in estimating generic distinctions, and, as 
shown above, cannot even be taken as specific; but at the 
same time there is always a number so far predominant in 
each species as to be characteristic of it, though, like the num- 
ber of arms in Starfish and the leaves of some plants, occa- 
sionally varying within certain limits. In the Nudibranchia 
this variation is most frequently .attributable to imperfect de- 
velopment. 
Through the whole of this genus the orange colour is pre- 
dominant; generally adorning the prominent parts in each 
species. This colour, however (passing on the one side into 
yellow and into scarlet on the other), is more or less prevalent 
throughout the family. Colour is sometimes characteristic of 
species in this group, but cannot always be relied upon. In- 
tensity of colour I take to be of no value. 
EXPLANATION OF PLATE IX. 
Fig.1, 2, 3. Polycera quadrilineata, in different positions. 
Fig. 4, 5, 6. Variety of the same. 
Fig. 7, 8, 9. Polycera citrina. 
Fig. 10, 11. Polycera cristata. 
Fig. 12. A branchial plume of the same. 
The whole of them are magnified; the lines opposite each indicating the 
natural size. 
