592 LECTURE XXIIT. 



the head, are exclusively attached to that part in the Nautilus, and 

 project from before the eyes and mouth. The mouth, besides its jaws 

 and spiny tongue, is now served by organs of prehension ; and it is 

 most interesting to observe that these cephalic preliensile tentacula, 

 at their first appearance, manifest the vegetative character in their 

 multiplied repetition and comparative simplicity, compared with 

 their homologues in the superior Cephalopods. 



Some of the Gastropods have a pair of jaws working upon each 

 other, but in the horizontal plane, as in insects : in the Nautilus they 

 are opposed to each other vertically, as in the Vertebrate series, and they 

 present a form which is repeated amongst fishes by the Scari, amongst 

 reptiles in the Chelonia, a,nd almost universally in the class of Birds. 

 The resemblance to the latter class which the Nautilus offers in the 

 modifications of the alimentary canal is also remarkable. 



In the very few conchiferous Gastropods that are able to swim, the 

 shell is of diminutive size, of a simple form and structure, and of an 

 extremely light and delicate texture. The strong and muscular oc- 

 cupant of the Pearly Nautilus-shell would not have been able, not- 

 withstanding its higher organisation, to have risen and swam on the 

 surface of the ocean unless it had been relieved from the impediment of 

 its large and dense abode by the introduction of some special mo- 

 dification in the testaceous structure. 



This is effected by the adoption, in the Nautilus, on a more definite 

 plan and larger scale, of the second mode of dealing with the part of 

 the shell successively vacated during the rapid growth of the animal, 

 which has been defined in the general account of the structure and 

 formation of univalve shells given in the foregoing Lecture.* The 

 abdominal part of the mantle of the Nautilus is attached to the 

 inner surface of the shell by the intervention of a horny or epithelial 

 cincture, the expanded lateral portions of which serve as the medium 

 of insertion of the adherent muscles of the shell : the cavity of the 

 shell posterior to this attachment is thus hermetically closed. A 

 third point of attachment is to the bottom of the shell by the pos- 

 terior extremity of the mantle,, which probably presents a conical 

 form in the embryo Nautilus. The line of attachment of both the 

 muscles and the cincture progressively advances with the growth of 

 the animal. A certain portion of the fundus of the shell thus becomes 

 vacated, and the Nautilus commences the formation of a new plate 

 for the support of the part of the body which has been withdrawn 

 from the vacated shell. The formation of the plate proceeds from 

 the circumference to the centre, and there meeting the conical pro- 



* P. 545. 



