220 LECTURES ON MOLLUSCA. 



food. The gill goes round both head and body, just under the shell ; 

 and is ornamented with very beautiful fringes, sometimes of two hun- 

 dred filaments. One of the south African Limpets, Olana, has a snout 

 in front of the shell ; but whether the animal has any coordinate pecu- 

 liarity, has not been ascertained. The shells which Messrs. Adams 

 call Cymbula are believed to be only True Limpets altered into a com- 

 pressed form to living on stems of plants. The Nacellce, or horny, 

 Sea-weed Limpets, alter in form in the same way. They have the gill 

 interrupted over the head, forming a transition to the Acmseids. The 

 shells of the African Helcion are like an Emarginula without slit. 



Fossil Limpets are found in rocks of all ages ; but of course their 

 generic position is uncertain. The Limpets, more perhaps than any 

 other shells, require to be studied geographically, with careful dissec- 

 tions of the animals, and with diligent comparison of a large multi- 

 tude of specimens. 



The last family of this order presents special characters so different 

 from any other mollusks, that if they alone were attended to, it would 

 be necessary to form a class for their sole occupation. Nevertheless, 

 they have so much in common with the Limpets that they are gen- 

 erally included in this order. 



Family Chitonid^e. (Coat-of-Mail shells, or Sea-iooodlice.) 



It has been well said that the Chitons have their backs armed, like 

 the Isopod Crustaceans; their gills, like those of the Brachyurous 

 Crustaceans ; their heart, in a long vessel down the back like a Sea- 

 worm ; their reproductive organs symmetrical and repeated on each 

 side, like the bivalves; a crawling foot and head, like a Limpet ; a pos- 

 terior vent, like the Fissurellas ; and a leathery skin, like the Tunica- 

 ries. According to the old-fashioned division of shells into univalves, 

 bivalves, and multi valves, they were driven by Linnaeus to keep com- 

 pany with the headless Pholas and the Crustacean Lepas. For they 

 have eight distinct shelly plates, fitting over each other like tiles, the 

 middle ones marked off in sculpture by diagonal lines, and all of them 

 let into the tough mantle by sharp smooth edges, like Pupillea. Out- 

 side, the creatures have a general resemblance to the bodies of Trilo- 

 bites; and, like those strange denizens of the palaeozoic seas, or the 

 living Woodlice, they can roll themselves completely up into a ball. 

 The eight valves and the skin together may be taken to represent the 

 shell of the Limpet. Underneath is a small head, with mouth, jaws, 

 and long armed tongue, the teeth being arranged in very peculiar 

 patterns. The young Chitons have very little resemblance to their 

 parents. They are divided into two nearly equal parts, head and 

 body, with a pair of eyes between. There is no trace of foot, gill, or 

 even mouth ; nor of the swimming fins almost universal in young 

 marine Gasteropods. They appear to change their fluids and grow 

 by suction, and to move by a fringe of feelers round the neck. Pres- 

 ently however the body' half develops lines on the back, between 

 which gradually seven of the valves are formed, the shelly matter first 

 appearing in granules, as in the land snails. At the same time a foot 



