134 KUDIBRANCHS. 



of Health, Thus small specimens of the Oyster and Mussel are 

 very serviceable in keeping down the number of the spores of the 

 sea-weed, which float in the water, and upon which theso 

 animals mostly subsist. In the same way the Periwinkles, and 

 the Trochi, or Top Shells, do good service in mowing down the 

 confervoid growth, which coats the glass and obscures all that is 

 passing within. The manner in which the Periwinkle sweeps off 

 this minute vegetation is extremely interesting, and has been 

 compared to the action of the tongue of an ox in licking up the 

 grass, or to that of a mower cutting swathe after swathe as he 

 passes along in the hay-field. The work is by no means cleanly 

 done, however ; and as the little Mollusc moves on, bringing the 

 white disc of his foot over the spot where he has been browsing, 

 the small square patches which have been swept by his tongue 

 are rendered very distinct by their contrast to the surrounding 

 parts that have not been touched. The most curious perhaps of 

 the Molluscs which readily accommodate themselves to the Aqua- 

 rium are the Chitons, which, clad in their coats-of-mail, adhere 

 like so many scales to the blocks of stone, although occasionally 

 they mount the glass sides of the vessel and help the Periwinkles 

 in clearing away the growing crops of confervoid plants. 



But by far the most attractive of the Mollusca are certain shell- 

 less animals popularly known as "sea-slugs," which, with a little 

 care and attention, may easily be induced to make themselves at 

 home in the Aquarium. To the naturalist these animals are 

 known as the Nudibranchiata, or naked-gilled mollusca, their 

 branchiae, or breathing organs, being open and exposed, arranged 

 symmetrically along the back in tufts, plumes, or variously-shaped 

 papilla?. The common name the Kudibranchs have won for 

 themselves, sufficiently indicates their general form, and their 

 mode of progression ; but let nobody imagine that these " sea- 

 slugs " have anything like the repulsive aspect of their namesakes 

 of our gardens and hedgerows. On the contrary, they are for the 

 most part more delicate and beautiful than words can well "He- 

 scribe, and when seen in a vase or tank of clear water, wandering 

 over the bright-green fronds of the sea-weeds, they call forth ex- 

 pressions of surprise and admiration from the most phlegmatic 

 and imimpassioned of observers.* 



* The British Nudibranchiata have been made the subject of a splendid 

 monograph by Messrs. Alder and Hancock, and the coloured illustrations of 



