70 



Genus 13. MAGILUS, De Montford. 



Animal; disc muscular , furnished posteriorly with a small ohlong 

 elliptic horny operculum ; head small, somewhat obscure, pro- 

 longed on each side into two short tentacles, on the outer side, 

 at the base, of which are the eyes ; proboscis obtuse. 



Shell ; ovate, spiral for three or four whorls, then continued in a 

 straight or flexuous direction to a considerable extent, the tube 

 forming a keel on the lower side and a corresponding siphon at 

 the aperture. 



The Magilus affords a strikmg example of the fallacy of arranging shells 

 according to their external form and aspect, without reference to the nature 

 and habits of their animal inhabitants ; it presents also a remarkable in- 

 stance of intelligent economy. Lamarck concluded from the vermiform 

 structure of this shell, that it must be formed by an Annelide, after the 

 manner of a Serpula, upon some foreign body, and he arranged it ac- 

 cordingly in tlie same natural group with the Testaceous Worms, the Earth 

 Worms, Leeches, &c. It was, however, discovered by Dr. Eiippell, during 

 his researches on the coast of Abyssinia, that the animal in question is a 

 true pectinibrancliiate moUusk, inhabiting masses of Madrepore; the 

 vermiform structure of whose shell is induced by its confined situation of 

 growth. The formation of the shell originates in the same spiral plan of 

 volution as that of other pectinibrancliiate moUusks, but the animal 

 fuiding it necessary to be in immediate communication with the sur- 

 rounding fluid, is obliged to leave its spiral plan of growth in order to 

 follow the enlarging surface of the coral. To accomplish this, it pursues 

 a straight, or flexuous growth, solidifying the posterior portion of the shell 

 in its progress, to enable it to reside witliin the vicinity of the aper- 

 ture''^; the soft parts of this moUusk scarcely exceed two inches in length, 

 although its shell is prolonged to the extent of from twelve to fifteen 

 inches, by a gradual slipping of the muscle of attachment along the colu- 



* Peu a peu la partie du corps de I'animal qui repose sur la coliunclle secrete en aboudance 

 de la matiere calcaire qui remplit non-seulement tout I'iiiterieur dcs premiers tours de spire, 

 mais qui force memo, par sa surabondance, ranimal a douner une autre direction aux nouveaux 

 toui-s de sa coquille, de maniere que celle-ci, qui jusqu'au quatrieme tour de spire est hcliciforme, 

 preud au-dela un accroisssement obsolument irregulier : tantot les nouveaux tours se contoiu'uent 

 en tire-bouchon, et tantot la coquille se continue en ligne presque droite ou coudee. On recon- 

 nait cependant toujoui's la cause de cet accroissement irregulier, qui est la copieuse secretion 

 de substance calcaire faite par le rebord gauche du manteau. II se forme par la une arete 

 emoussee le long de la gouttiere de I'ouverture, et la partie des nouveaux tours qui reposerait sw 

 la columelle pi'end tantot une siu'face lisse, et tantot elle prcsente des lames irregulieres. Pendant 

 que la coquille s'alonge ainsi par son accroissement, la cavitc dcs premiers tours se remplit pro- 

 gressivement de maniere que la cavite de la coquille dcpassc rareincnt deux pouccs de profoudeur, 

 quoique le tube entier ait souvent dome a quiuze pouces." — Euppkll, Ilhn. 1832. 



