368 



MOLLUSCA. 



the madrepore ; and, increasing itself in size and length as the madrepore increases around it, it kjeps 

 the aperture even with the outer surface of the coral, and thus grows, in some instances, to a consi- 

 derable length. This singular testaceous parasite is common in the coral rocks of the Isle of France, 

 and its tube sometimes reaches the length of three feet.] 



Siliuuaria, Drug. — 

 Resembles Vermetus in the head, the position of the operculum, and in the tubular and irregular shell; 

 but there is a fissure on the whole length of the shell which follows its contour, and which corres- 

 ponds with a similar cleft in that part of the cloak which covers the branchial cavity. Along the 

 whole side of this cleft is a branchial comb, composed of numerous delicate and tubular-like leaflets. 

 Linnaeus left these shells also in Serpula ; and until a very recent date they were believed to be mem- 

 bers of the class Annelides. [The remarkable operculum is similar to the pod of a Medicago, consisting 

 of a spiral lamella rolled five times round an axis like a pulley. This horny lamella is very lustrous 

 underneath, farinaceous or subpubescent above, and subcrenate on the under side of the rim, 

 with short striolae. It is convex in the centre, and the projection is multilocular, very exactly resem- 

 bling a Cristellaria or Robulina.] 



THE EIGHTH ORDER OF THE GASTEROPODES. 



THE SCUTIBRANCHIATA.* 



The order comprises a certain number of Gasteropods having a considerable resemblance to 

 the Pectinibranchiata in the form and position of the branchiae, as well as in the general form 

 of the body, but they are complete hermaphrodites. Their shells are very open, without an 

 operculum, and the greater number are not in any degree spiral, so that they cover their 

 animals, and particularly the branchiae, in the manner of a shield. The heart is traversed by 

 the rectum, and receives the blood by the two auricles, as in the majority of the Bivalves. 



The Haliotides {Haliotis, Linn.) — 

 Are the only family of this order in which the shell is turbinated ; and from those shells it is distin- 

 guished by the excessive amplitude of the aperture, and the flatness and smallness of the spire, which 

 is seen from within. This form has caused it to be compared to the ear of a quadruped. 



In the Haliotis, Lam., the shell is perforated along- the side of the columella with a series of holes ; and when 

 the last hole remains incomplete, the shell has the appearance of being 1 emarginate. The snail is one of the most 

 richly adorned of Gasteropods. A double membrane, with a furbelowed margin, and furnished with a double row 

 of filaments, extends, at least in the commonest species, round the foot, and on to the month : outside its long 

 tentacula are two cylindrical pedicles, which support the eyes. The cloak is deeply cleft on the right side, and the 

 water, which passes through the holes of the shell, gains access, by the medium of the cleft, to the branchial cavity. 

 Along the margins of the cleft there are also three or four filaments, which the animal can also protrude through 

 the holes of the shell. The mouth is a short proboscis. 



Padolla, Montf. [Stomatella, Lam.] has an almost circular shell ; almost all the holes obliterated ; and a deep 

 groove that follows the middle of the whorls, and shows itself exteriorly by a corresponding ridge. 



Slomatia, Lam., have a more concave shell, with a more prominent spire, and without holes : they otherwise 

 resemble the Haliotis,and connect that genus with certain kinds of Turbo. The animal is less adorned than Haliotis.f 



The following genera, dismembered from Patella, have the shell quite symmetrical, as well as the posi- 

 tion of the heart and branchiae. 



Fissurella, Lam. — 

 Have a broad, fleshy disk under the belly, as the Patella ; a conical shell placed over the middle of the 

 back, but not covering it completely, and perforated in the summit with a small aperture, which serves 

 both for the passage of the excrements, and of the water necessary to respiration : that aperture pene- 

 trates into the cavity of the branchiae situate over the front of the back, at the bottom of which the anus 

 opens ; and this cavity is moreover widely patulous over the head. There is a branchial comb on each 



* M. de Blainville unites this and the following order in his sub- 

 class Paracepkalophora herynaphrodita. 



t [Padulu and Slomatia (that constitute but one genus, according 

 to Sowerby,} are placed in the order Pectinibranchiata by Rang, where 

 we find also next them the I'elutina of Hemming, distinguished by its 



neritoid thin shell with a wide ent're aperture, without an operculum 

 His Stylina [Stylifer, Broderip) has also no operculum, but the spire 

 is pointed and acute. One species lives on the Echiut-s ; another im- 

 beds itself in Starfish.] 



