MOLLUSCA. 



177 



characteristics it is not our intention to enter ; still less do we 

 purpose giving any enumeration of. the genera into which the 

 several orders are subdivided. We shall merely endeavour to 

 convey some idea of the principles on which the classification 

 is conducted, and relate some particulars with regard to the 

 habits, structure, or uses of a few well-known species. 



In two orders the animals are all marine, and are destitute 

 of any shelly covering. In that to which the term Nudi- 

 branchiata is applied, the gills are also naked or unprotected, 

 and are arranged in various forms, and attached to different 

 parts of the body. The animals are found upon the rocks and 

 seaweeds on our shore, and floating with the foot uppermost, 

 on the smooth surface of our bays ; they are 

 also dredged up from considerable depths. 

 When placed in sea- water, they exhibit figures 

 of great delicacy, variety, and elegance, and 

 with a beautiful diversity of colouring. Their 

 size is very different, some of our native species 

 being less than half an inch in length, while 

 others measure so much as four inches.* The 

 eggs of many are in the form of a delicate 

 spiral ribbon-shaped coil, and are attached to 

 stones near the shore or to corals in deep 

 sea-water, according to the habits of the 

 species.f Some gaily-coloured members of 

 this group are found in the Mediterranean 

 and the Indian seas, and swim with great 

 rapidity. 



The common Limpet forms an example of 

 a Mollusk of a different order, in which the 

 gills extend like a fringe round the lower edge 

 of the body, and between the body and the foot 

 ( Cyclobranchiata). Those who see the Limpet 

 only when left uncovered by the tide have no idea of the ease 

 with which it can march about when the returning waters 

 once more surround its dwelling. Its little excursions are not, 

 however, "idlesse all;" they are undertaken for the important 



* R. Ball. Vide W. Thompson, on Molliisca of Ireland, in Annals 

 of Nat. Hist. 1840. 



f Fide an elaborate Monograph on the British Species of Nudibranchiate 

 Mollusca, by Messrs. Alder and Hancock, now in course of publication 

 by the Ray Society. It is illustrated with figures of exquisite delicacy. 



Fig. 164. EOLIS. 



