AND CLASSIFICATION OF THE HETEROPODA. ] 1 



tor, is fixed, so as, probably, to assist in forming a vacuum when the animal is 

 entirely supported by this small disc ; and the epithelial cells on its outer surface 

 are frequently tinted with a yellowish, red, or purple pigment. 



The metapodium bears the operculum (Plate II. fig. 1, A, and fig. 1'.) on its 

 dorsal surface, and terminates in a point posteriorly. The inferior surface of this 

 curiously constructed tail-piece presents a median vertical fold or reel, which gra- 

 dually decreases in depth as it approaches the pointed extremity where it ends. On 

 either side of this fold is a little fossa, deepening in front, and passing some little 

 distance within the anterior margin. To this latter part is appended a small 

 moveable and valve-like process (Plate 11. fig. 1. <), whose use I have not yet dis- 

 covered ; but it is probable that it has some office in directing the path of the 

 respiratory currents, when the animal is retracted into its shell ; and this view 

 is favoured by the fact that the floor of each fossa is richly ciliated, with the un- 

 dulations proceeding from behind forwards. 



Organs of Sense and Nervous System. — The eyes (Plate II. fig. I, e), as above- 

 mentioned, are proportionately very large, and situated immediately above and in 

 front of the cerebroid ganglia, capped over by a portion of common integument, 

 which bulges out to enclose them. The outer and fore-part of this covering is 

 beautifully rounded, smooth, and transparent, so as to form a kind of external 

 cornea, lying directly over a brilliant spherical lens. Commencing at a zone cor- 

 responding with the limits of the cornea, the cell-pavement of the integument, 

 and the deeper muscular tissue, become more apparent; and in front of each 

 cornea, towards its inner side, is a conical and contractile tentaculum (Plate II. 

 fig. 1, c) of small size compared with that of the eye. The lens, which is com- 

 pressible to a certain extent, and invested with a capsule of homogeneous mem- 

 brane, lies in front of a cylindrical case, which is somewhat fuller at the inner 

 and posterior part, where the optic nerve expands into the retina, and all stray 

 luminous rays are absorbed by a dense coating of reddish-purple or black pigment 

 cells extending forwards to the lens, upon which it encroaches in an annular form 

 to about one-fourth of its diameter. 



At the inner side of the base of each eye there is a small projection upon 

 which it rolls, obedient to the action of numerous small muscular bands that 

 spring from the borders of a kind of socket, and are inserted into the organ at 

 different points of its circumference. Thus the eyes enjoy a considerable range 

 of motion, quite independent of the outer covering ; and often, when the bright 

 lens rolls within the margin of the pigmentary coating, the singular appearance 

 of winking is produced. Both eyes generally move in concert, but each organ 

 enjoys its own intrinsic movements irrespective of the other. 



The acoustic capsules (Plate 11. fig. 1, g), consisting of homogeneous membrane 

 with a ciliated lining, contain each a single vitreous-looking otolith, usually with 

 a small cavity in its centre, around which the successive deposit of concentric layers 



