716 



CONTRACTILITY. 



This tabular view of the classification exhibits 

 certain particulars, upon which we deem it 

 necessary to offer a few explanatory remarks. 

 As we said before, the series as a whole may 

 be regarded as a common trunk, from which 

 various branches spring, sometimes anastomo- 

 sing, sometimes ending abruptly. It is thus 

 that from the Clavigella we observe a lateral line 

 departing, formed of the genera Fisttdana, 

 Gulcomma, and those of Lamarck's family 

 of Petricola. These genera descend parallel to 

 the common trunk of the classification, so as 

 to approximate in as great a degree as possible 

 the genus Venerupls to the genus Venus. The 

 genus Pandora has numerous analogies on one 

 side with the Corbula, l*ut it lias also many 

 with the members of the genus Osteodesma, on 

 which account it is made to depart laterally 

 from the Corbula, and to ascend towards the 

 Osteodesmata. The Luiraria are also variously 

 related to several genera of the Osteodesmala, 

 and this genus is joined to that of the Thracia 

 by means of the genus Anatinella, which we 

 place crosswise to connect the genera just men- 

 tioned. In the Mactracea, we pass without 

 any very great stride from the Luiraria to the 

 Mactra, from the Mactra to the Erycina 

 and to the Amphidesma. Farther, in order 

 not to interrupt this series of relation- 

 ships, we place upon a lateral line departing 

 from the Mactra the two genera Mcsodcsma 

 and Crassatella. Every naturalist knows how 

 great the resemblance is between the flat and 

 broad Solens ( Soletcllina, De Blainv.) and the 

 Psummobice ; but we also know that the genus 

 Psammubia has so many analogies with the 

 family of the Tellinida, that it is impossible to 

 detach it from this family in order to include it 

 within the family of the Solenaceae. To avoid 

 interrupting the relations of this genus to those 

 of the Solcn family, we have recourse to an 

 ascending line composed of the genera Solen- 

 ertus, Panopaa, Sulen, Solemya, and Glyci- 

 meris, by which means we approximate, as 

 much as possible, these last genera to the fa- 

 milies Pholadia and Osteodesrnata, with which 

 they have in fact unequivocal relationships in 

 point of organization. We consider the family 

 of the Lucinidae as a lateral and truncated 

 branch of the Conclude, divaricating from the 

 genus Astarte. With regard to the Cycladn, we 

 place the genus Glaucoma of Mr. Cray laterally, 

 between the Cyrcnas and the Vcnuses, so as to 

 establish the connexion between the two ge- 

 nera; whilst departing also from the genus 

 Cyrena we place our genus Cyrenclla obliquely 

 in order to make it join that of Lucina, 

 this genus of Cyrenella being to Cyrena 

 and Lucina that precisely which the genus 

 Glaucoma is to Cyrena and Venus. To 

 us the family of the Chamacea is a lateral 

 offset from that of the Cardiaceu, and although 

 the Elheria and the family of the lludisies are 

 in reality among the number of those Conchife- 

 rous mollusca which have the lobes of the 

 mantle disjoined, still as they do not imme- 

 diately arrange themselves in any particular 

 part of this section, we have placed them to the 

 side in continuation of the family of the Cha- 



macea, but underneath them. The family of 

 the Ostracea we now believe to consist of the 

 single genus Ostrea, and we propose under the 

 name of Placunida a family containing the 

 three genera P lacuna, Placunonomia, and Ano- 

 mia, which according to our views constitute a 

 descending and lateral line really intermediate 

 to the Conchifera and the Brachiopoda. 



BIBLIOGRAPHY. The following works and 

 essays may be referred to as still interesting 

 on the natural history of the Conchifera. Reau- 

 mur, De la formation et de Faccroissement 

 des coquilles, Acad. de Paris, An 1709 et An 

 1716; Ej. De la maniere dont plusieurs especes 

 d'animaux de mer s'attachent au sable, aux pierres, 

 &c. ibid, An 1711. Wulch, Vom Wachsthum und 

 den Farben der Konchilienschaalen. Besch. der 

 Berlin. Naturforsch. Gesell. B. i. S. 230. Miiller, 

 Anmerk. ueber Walch, ibid, B. ii. S. 116. * * 

 Cuvier, Nouvelles Rech. sur les coquilles bivalves, 

 Societe Philomath. A. 7, p. 83. Lister, Anatomy 

 of the Scallop, Philos. Trans. Year 1697. Ant. 

 van der Heide, Anatome Mytuli, 12mo. Amst. 

 1684. Bojanus, Sendschreiben an G. Cuvier 

 iiber d. Athmen und Kreislaufswerkzeuge d. Zwei- 

 shaaligen Muscheln, 4to. Jena, 1820". Mangili, 

 Ricerche nuovi zootomiche sopra alcune specie di 

 Conchiglie Bivalvi, 8vo. Milano, 1804. Brack, 

 De ovis ostreorum, Misc. Ac. Nat. cur. Dec. 2, 

 An 8. Koehlreuter, Obs. anat. physiol. Mytili 

 cygnei (Lin.) concernentes, Nov. Act. Petrop. 



(G. P. Des Hayes.) 



CONTRACTILITY. Since it has been 

 generally understood, that all the most striking 

 and conspicuous movements which take place 

 in living animals, depend on peculiar contrac- 

 tions of certain of their solids, the circum- 

 stances of these contractions, the causes by 

 which they are excited, and the laws by which 

 they are regulated, have been justly regarded 

 as objects of the highest interest, and of fun- 

 damental importance, in physiology. The 

 term Irritability was employed by Haller and 

 his followers, to denote all such contractions 

 in living bodies, as they judged to be peculiar 

 to the living state ; but more recent inquiries 

 have shewn the necessity of distinguishing 

 different species of these contractions ; and the 

 more comprehensive term Contractility is now 

 pretty generally employed. To this the epithet 

 Vital, in physiological discussions, may usually 

 be understood as prefixed. 



It is to be remembered, however, that several 

 of the animal textures are endowed with a pro 

 petty of contraction, in certain circumstances, 

 which is not peculiar to their living state, but 

 subsists as long as their structure remains unal- 

 tered after death ; and the distinction between 

 the phenomena resulting from this cause, and 

 those which are strictly vital, has not always 

 been accurately observed. Thus many of the 

 soft animal textures, muscles to a certain de- 

 gree, tendons and ligaments in a greater degree, 

 and arteries in a still greater degree, are elastic, 

 and liable to contractions from that cause when 

 stretched. The Contractility de Tissu of Bichat 

 is in most cases to be considered simply as 

 Elasticity, although in some cases (as when he 

 assigns this property as the reason of the re- 

 traction of the cut extremities of a living muscle 



