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. I, 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. 



