70 



Genus 13. MAGILUS, Be Montford. 



Animal; disc muscular, furnished posteriorly with a small oblong 

 elliptic horny opercidum ; 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 directio?i to a considerable extent, the tube 

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

 the aperture. 



The Magilus affords a striking 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 tins 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 the same natural group with the Testaceous Worms, the Earth 

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

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

 true pectinibranchiate mollusk, 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 pectinibranchiate mollusks, but the animal 

 finding 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 within the vicinity of the aper- 

 ture*; the soft parts of this mollusk 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 l'animal qui repose sur la colunielle secrete en abondance 

 de la matiere calcaire qui remplit non-seulement tout l'interieur des premiers tours de spire, 

 niais qui force meine, par sa surabondance, l'animal a douucr une autre direction aux nouveaux 

 tours de sa coquille, de maniere que celle-ci, qui jusqu'au quatrieme tour de spire est heliciforme, 

 preud au-dela un accroisssement obsolument irregulier : tautot les nouveaux tours se contournent 

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

 nait cependaut toujours 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 

 enioussee le long de la gouttiere de Pouverture, et la partie des nouveaux tours qui reposerait sur 

 la columcllc prend tantot une surface lisse, et tantot elle preseute des lames irregulieres. Pendant 

 que la coquille s'alonge aiusi par son accroissement, la cavite des premiers tours se remplit pro- 

 grcssivenient de maniere que la cavite de la coquille de'passe rarcment deux pouccs de profoudeur, 

 quoique le tube cutier ait souvent douze a quinzc pouccs." — Rupfell, Mem. 1832. 



