130 GLAUCUS; OR, 
likeness. I am half jesting; that cannot be the 
only reason, perhaps not the reason at all; but the 
fact is one of the most curious, and notorious also, 
in comparative anatomy. 
Look, again, at those sea-slugs. One, some three 
inches long, of a bright lemon-yellow, clouded with 
purple; another of a dingy grey ;* another exquisite 
little creature of a pearly French white,” furred all 
over the back with what seem arms, but are really 
gills, of ringed white and grey and black. Put that 
yellow one into water, and from his head, above 
the eyes, arise two serrated horns, while from the 
after-part of his back springs a circular Prince-of- 
Wales’s-feather of gills—they are almost exactly 
like those which we saw just now in the white 
Cucumaria. Yes; here is another instance of the 
same custom of repetition. The Cucumaria is a 
low radiate animal—the sea-slug a far higher mol- 
lusc; and every organ within him is formed on a 
1 Doris tuberculata and bilineata. 
2 Eolis papi losa. A Doris and an Eolis, though not of these 
species, are figured in Plate X. 
