VERMES. 



draw the foot and the whole body within the (hell. They 

 arife from the colLnnella or axis of the (hell, and, having 

 penetrated the body below its fpiral part, they run forward 

 under the flomach, and fpread their libres in feveral flips, 

 which interlace with thofe of the mufcles proper to the foot, 

 the fubftance of which they enter. From thefe attachments, 

 their mode of aftion may be eafdy underftood. When the 

 animal wilhes to protrude itfelf from the (hell, its head and 

 foot are forced out by circular fibres, which furround the 

 body immediately above the foot. 



The acephalous moUufca have the body enveloped by a 

 membrane principally mufcular, which is called the mantle 

 or cloak. This integument is more or lefs complete in the 

 different genera. It is generally covered by valves or (hells 

 of various forms and proportions. Few of the genera want 

 this folid covering ; among thofe, however, are the afcidia 

 and falpa. 



The valves of the (hells are fo difpofed, that they can 

 move one upon the other, by means of o(reous projedlions, 

 which reciprocally receive each other, thus forming a real 

 hinge. They are, befides, connefted by an elaftic ligament 

 of a horny fubftance, which continually tends to open them. 

 " This elaftic fubftance," fays Mr. Carlifle, " is wedged in at 

 the hinge : its fpring is excited by comprefTion ; but it does 

 not pofTefs the property of expanfion beyond its paffive ftate. 

 When dried, it cracks into cubes. As the valves increafe, 

 this elaftic ligament is augmented along the inner furface 

 only, and muft have been always depofited daring the ex- 

 panded ftate of the valves, fmce the limits of its elaftic con- 

 dition are exaftly adapted to that ftate. As the lamina of 

 the (hells increafe, there is a gap at the outfide of the hinge, 

 filled with foft crumbling and decompofing worn-out elaftic 

 ligament : this gap prefents two inclined planes meeting at 

 an acute angle, and that fpace is kept free from pebbles and 

 hard extraneous bodies by the prefence of the decompofing 

 ligament ; as fuch an accident would prove fatal, by pre- 

 venting the opening of the valves." Monthly Repofitory 

 for Auguft, 1 8 15. 



The hinge of the (hells prefents fo many varieties, that 

 naturalifts have drawn from it the charafteriftics of the 

 genera. The oyfter, placuna, fcallop, avicula, &c. have 

 no tooth in their joint. The piddocks and the mya or 

 gapers have it in one of the valves only ; but it is not received 

 into a foffa. The razor-(hells have the hinge ftrengthened 

 by a tooth in each (hell, which projefts inward. Thefe 

 two projeftions meet and move upon each other. The 

 anomia, unio, chama, fpondylus or thorny oyfter, and fe- 

 veral others, have one or two teeth on one valve only, 

 which are received into correfponding cavities, in the op- 

 pofite valve. The venus, cockle, and maftra, have teeth 

 on each (hell, which are mutually received. Laftly, the 

 area has a multitude of httle teeth, which are clofely in- 

 dented with each other. Thefe different conformations 

 either facilitate the motion of the hinges, or ftrengthen 

 the joint ; or they permit a greater or lefs opening of the 

 valves. 



The elaftic ligament, which tends continually to open the 

 valves, is not always fituated at the fame point of the (hell. 

 The mufcles, for example, have it at one fide of the valves. 

 The placunx have a little ofleous appendage, which forms a 

 projeftion in the infide of each valve ; and from this arifes 

 the ligament that holds them together. The 'perna has in 

 each valve feveral Lttle cavities, oppofite to each other in 

 pairs, in which an equal number of fmall ligaments are 

 Jodged. 



The ftiells of the acephala prefent feveral other pe- 



culiarities. We find the valves immoveable, and foldered to- 

 gether at the angle, in the pinna. The teredo or pipe-worm 

 has the body inclofed in a calcareous tube, and is armed 

 with tv^'o little moveable valves, which are ufed in pene- 

 trating wood. The terebratula has on the inner part of one 

 of the valves two olTeous appendages, which fupport the 

 body. 



The contraftile membrane which covers all the body of 

 the acephalous moUufca, and is called the mantle, is a real 

 mufcle, prefenting many varieties. Sometimes, and indeed 

 moft commonly, it is open before, in the direftion of the 

 valves, as in the oyfter, the mufcle, &c. ; in the (hells that 

 have two ends always open, as in the razor-fliells, the 

 gapers, the piddock, &.c. it is perforated at both extremities. 

 Laftly, the cloak may envelope the whole body of the 

 animal, and be open at one end only, as in the afcidia. 



The cloak of the oyfter is compofed of two pieces of the 

 fame form as the (hell ; they are fixed to the body pof- 

 teriorly, or on the fide of the hinge, and extend to the 

 edges of the valves. Their fubftance is foft, femi-tranf- 

 parent, and furnifhed with a number of mufcular bands : 

 they are perforated by the mufcle, which clofes the (hell. 

 One of the edges is in folds, like a flounce, and feftooned ; 

 the other is furni(hed with fmall conical and contraftile 

 tentacula. The cloak of other acephala differs from this 

 defcription in its general form ; in the tentacula on its edge ; 

 in the tubes, which are prolongations of it ; and, laftly, in 

 the mufcles which perforate it. 



The aperture which ferves for the expulfion of the feces, 

 and that which receives water and the different aliments, are 

 fometimes prolonged into a kind of tube, which is a con- 

 tinuation of the cloak : this is called a probofcis (in French 

 " trompe.") The oyfter, the mufcle, the unio, the ano- 

 dontites, have only one of thefe apertures, which is the 

 anus. The water merely enters by the large flit in the 

 cloak. In the cockle, each aperture is a few lines elon- 

 gated ; that which ferves for refpiration is longer and larger 

 than the other. They are ftill more elongated and unequal 

 in the venus, tellina, maftra, and fome other genera. The 

 razor-fhell has likewife two ; but in the piddock, both tubes 

 are inclofed in a very thick flefhy probofcis, through the 

 whole length of which they pafs without uniting. 



In the acephala that have the cloak open before, the 

 tentacula are placed at its edge, and in particular towards 

 the anus ; but in thofe which have tubes, they are fituated 

 at the orifice of the probofcis. In the edible mufcle (my- 

 tilus eduhs, Linn.), they are branched. 



The valves of (hells having a continual tendency to open, 

 in confequence of the aftion of the elaftic ligament fituated 

 at the fide of the hinge, it was neceffary that the contained 

 animal (hould have the power of clofing them at pleafure. 

 For this purpofe they are furnithed with mufcles, palling 

 between the valves at right angles. In the oyfter there is 

 only one mufcle of this kind, fituated near the centre of the 

 fheil, behind the liver, and in the middle of the cloak. It 

 is equally inferted into both valves, palling in a ftraight 

 line between them ; and bringing them together, by its f.on- 

 traftion, with an aftonilhing force. In the moderate fepa- 

 ration of the valves, we obierve the operation of the elaftic 

 ligament, when the mufcle is relaxed : if we tough the ani- 

 mal, the (hell is inftantly clofed ; and we can eftimate the 

 power with which this is accomplilhed, by the amount of the 

 force required for the forcible difruption of the valves. 

 The fame mechanifm is feen in the perna, avicula, and 

 fpondylus. 



There are two mufcles for clofing the (hell in the mvtilus, 

 8 'folen. 



