Patellid.e.- 



-MOLLUSCA.- 



-Chitonid.e. 



317 



adliesiun, " Thoy adhere very firmly by atiiiosijlieiic 

 jiressiire (lifteeii pounds per square iiicli), and the difli- 

 culty of detaching them is increased by tlie form of tlie 

 th<:\\."— (Woodward.) Dr. Johnston, on the strength 

 of Reaumur's experiments upon this subject, affirms 

 tiiat tliis coliesion entirely depends on a glue, or kind 

 of paste, which, though invisible, produces a very con- 

 siderable effect. Reaumur cut the animal from top to 

 bottom in two halves, as it stood fixed perpendicularly 

 upon the rock ; and he made other deep incisions in a 

 horizontal direction, destroying in this manner all the 

 muscular power of its base, and all supposable vacuity 

 between it and lliJ stone : but the adhesion continued 

 as firm as before the experiment. Even the death of 

 the aidmal does not destroy its cohesion. There seems 

 to be some doubt as to how far these animals are sta 

 tionary on the rocks, or whether they have the jiower 

 of moving to some distance and returning to their roost. 

 Mr. Clark, in his " Molhisca," says, " IIow some Patellae 

 live is a mystery;" they are often fixed for months, 

 [jerhaps years, on rocks, at altitudes where they can 

 rarely, if at all, be aspersed by the sea, and are debarred 

 access to marine vegetables ; their recorded descents 

 from high levels, and periodical exits from and returns 

 to the identical hollows they have made, after feeding 

 on alga?, have almost a fabulons complexion ; zones of 

 sand fifty yards wide often intervene between them and 

 such food, and their exceeding slow locomotion is op- 

 posed to such manceuvres. Mr. Lukis of Guernsey, 

 however, in a paper published in London's Magazhu, 

 has apparently shown that these sluggish creatures 

 have tlie power, and exercise it too, of moving to some 

 distance from their resting-place and returning to the 

 same favourite spot. The times for these excursions 

 lieing during night, and when the rocks are covered by 

 the tide, his led superficial observers to suppose that, 

 because the same individuals are often found in the 

 same spots for a great length of timo, months or even 

 years, thoy therefore never leave these sites at all. Mr. 

 Lukis appears from his paper to have marked indi- 

 viduals, to avoid mistake, and has watched their roam- 

 ings and followed the tracks left bj' them for several 

 yards. Tliese tracks are very peculiar, and when once 

 seen are not to be mistaken. 



This Common limpet, of which we have spoken so 

 much (see fig. annexed), is much used as an article of 



Fig. 22G. 



Patella vulgata and animal. 



food. In the north of Ireland, ]\[r. Patterson gives a 

 very interesting account of the use made by the poor 

 of this, to them, valuable article of subsistence. Many 

 tons weiglit are annually collected near the town of 



Larne alone. In Guernsey, Mr. Lukis tells us, great 

 quantities are daily consumed ; " scarcely a cottage on 

 the coast is seen, wdiere a heap of empty shells does 

 not form a prominent feature near the door." Patella 

 vulfjata, lie adds in a note, " seems to have been used 

 by the inhabitants of these islands from the earliest 

 times, as appears from masses of its shells now found 

 in ground which has lain waste and unturned for cen- 

 turies." 



This shell is much used by fishermen as bait. In 

 the Berwickshire Club Transactions, Dr. Johnston in- 

 forms us tiiat, on the coast near Berwick, there were for 

 many years nearly twelve million collected yearly fur 

 that purpose ; but they have decreased in number so 

 much of late that they no longer repay the trouble of 

 gathering them. 



F.vMiLY— CIIITONID^E {Sea Wood-lice). 



In this family the shell consists of a series of eight 

 transverse valves situated on the middle of the animal's 

 back, each valve being inserted into the mantle, and 

 having a deep lateral notch on each side. The border 

 of the mantle into which these valves are inserted, is 

 of a coriaceous texture, and is either sruooth and bare, 

 or covered with minute scales, sjiines, or hairs. There 

 are several anatomical peculiarities possessed by the 

 animals of this family, of such a nature as to induce 

 some naturalists to remove them from the Mollusca 

 altogether. "A Chiton," says Dr. Williams, "has a 

 carapace like an isopod crustacean ; a dorsal vessel like 

 an annelid; bilateral, symmetrical, reproductive viscera 

 like an acephalous mollusc; a head and foot like a patel- 

 loid gasteropod; a posterior anus like the FtsntrellidiV, 

 and branchia; like those of the brachyurous Crustacea I 

 Such manifold afiinities at once unite and sever this 

 odd group from several most dissimilar classes." As 

 Cuvier, however, has shown, their gills are like those 

 of the Putellida; their foot is that of a tine gasteropod, 

 and in their lingual dentition they resemble the Cteno- 

 hranchiala. The hinder valve of the shell was at one 

 time considered by Dr. Gray as homologous with the 

 shell of the Palellidce, the other valves being only so 

 many portions successively detached from it. The 

 species of Chitonkla; are very nuiuerous, more than two 

 hundred having been described. They are world-wide 

 in their distribution, occurring in all climates. Though 

 by far the greater number are found on rocks at low 

 water, they are occasionally taken by the dredge in 

 from ten to twenty-five fathoms water ; and some of 

 our small British species range as low as one hundred 

 fathoms. " In the tropics," says Mr. A. Adams, " the 

 Chitons appear to be more vivacious than those found 

 firrthcr north. If turned over on their backs, they will 

 gradually bond their calcareous jointed bodies in every 

 direction, contracting and dilating their ventral disk 

 until they assume their natural position. Their pro- 

 gressive motion is scarcely perceptible, however; the 

 principal object, apparently being again to fix them- 

 selves to the surface of the rocks which Nature has 

 given them to inhabit. Their food consists of fuci and 

 other algaB with which the rocks and stones are covered." 

 In the West Indies there are several species which grow 



