118 



keep up the family 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, wdiile 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 mollusc ; and eveiy organ 

 within him is formed on a different type ; as indeed 



* Doris tuberculata and bilineata. 



f Eolis papillosa, A Doris and an Eolis, though not of these 

 species, are figured in Plate X. 



