198 



NATURAL BISTORT. 



Upwards of one hundred and forty species of Pur pur a are known and described ; they are 

 almost world-wide in their distribution, and extend from low water to twenty-five fathoms. 



The " Unicorn-shell," Monoceros, is peculiar to the West coast of America, whence eighteen 

 species have been brought. The shell is like that of Purpura, but with a spiral groove on the 

 whorls, ending in a prominent spine, or tooth, at the lower or anterior end of the outer lip. 



Mayilus is a truly remarkable Molluscan genus, living parasitic, and 

 boring in live Coral in the Red Sea, and on the coasts of Mauritius and 

 Java. When young, the shell of Magilus, like any ordinary sea snail, is spiral 

 and thin, with its aperture channelled in front; as it grows older, the shell 

 ceases its spiral growth, and is prolonged instead into an elongated, irregular 

 tube, at the extremity of which the animal resides, having in its onward growth 

 filled up its original spiral shell and the greater part of the tube with solid 

 shell-matter, compact and somewhat translucent, like aragonite. Formerly it was 

 believed that the Magilus, which lives fixed in the living Coral, grew upward 

 with the growth of the zoophytes in which it becomes immersed ; but from speci- 

 mens obtained, imbedded in the Coral itself, Mr. Charlesworth has shown that 

 Magilus grows horizontally, eating its way through the Coral near the living 

 surface, so as probably to reach and devour the zoophytes within, and yet always 

 to remain concealed until the mass of the Coral is cut open, exposing the tortuous 

 and solid, but once tubular shell. It is interesting to notice that in every case 

 in which any animal, say a Crustacean or a Mollusc, becomes parasitic, it 



MAGILUS ANTIQUUS A i nvar% i a bly loses some of its organs by disuse, and becomes malformed. Witness 



YOUNG ; B, ADULT, the " Hermit-crabs " living in sea snails' shells ; the Teredo in timber ; the 

 Siliquaria in Sponges ; and the Magilus in Coral, and many others. When we 



consider that the coral-boring Magilus advances through the Meandrina, or other compact Coral, not by 

 the movement of its shell, but by the slow, onward growth of the animal itself, eating its way through 

 the living Coral mass, we can the more readily understand whence it obtains such a store of 

 lime, sufficient to enable it to fill up the deserted earlier portion of its shell with so compact 

 and solid a mass of crystalline material. In the British Museum is a Coral in which 

 a Magilus has resided for a long time, and traversed the mass in a tortuous manner, 

 leaving its solid tube behind. In one place, a LitJiodomus, a bivalve Mollusc, also in the habit of 

 making burrows into the same Coral, has driven its shaft at right angles to the tube of the Magilus, 

 and has cut its tunnel right through the solid por- 

 tion of the shell of the Magilus, regardless of its 

 greater density. The tube of Magilus is sometimes 

 as much as fifteen inches in length, and very heavy. 

 The animal has a concentric lamellar operculum, 

 with its nucleus near the outer edge. Only one 

 species has been described, the Magilus antiquus. 



The " Harp-shells " (Harpa), so called from 

 the numerous sharp, smooth ribs placed at regular 

 intervals on the surface of the shell, like the 

 strings on a harp, form a group of elegantly-marked 

 and coloured shells. The shell is ventricose, the 

 spire is small, the body-whorl and aperture of shell 

 large, and notched in front. The animal has a very 

 lai-ge foot, with the front crescent-shaped, and 

 divided from the posterior part by deep lateral fissures, which are said to separate spontaneously when 

 the animal is irritated. It has no operculum. Nine species are described, all of which are tropical. 

 Harpa lives in deep water, on soft, sandy, or muddy bottoms. 



The " Olives " (Oliva) are a numerous family, all with cylindrical, highly-polished, often very 

 prettily marked and coloured shells. The spire is very short, the suture is channelled ; the aperture 

 is long, narrow, notched in front; the columella is thickened and obliquely striated; and the body- whorl 



HARPA IMPEHIALIS. 



HARPA ARTICULARIS. 



