GASTEROPODA. 



Fig. 193. 



395 



by for too small to admit of any satisfactory 

 examination of tlicir internal structure; and even 

 in the. largest forms of the organ which are met 

 with in the more bulky marine genera, it is wiili 

 difficulty that their organisation can be explored. 

 In fig. 193 we have delineated the ]x>sition 

 and structure of the eye in a large Murex. 



Fig. 193. 



Structure of the trutackt in the Garden- Smut 

 ( Helix Pomatia). 



From the common retractor muscles of the 

 foot four long muscular slips are detached, 

 one for each horn ; these run in company with 

 the nerve to each tentacle, passing within its 

 tube when protruded, quite to the extremity 

 ((!). The contraction of this muscle dragging 

 the apex of the organ inwards, as seen at r, of 

 course causes its complete inversion, whilst its 

 protrusion is effected by the alternate contrac- 

 tions of the circular bands of muscle of which 

 the walls of each tentacle are composed. There 

 is, however, another peculiarity rendered neces- 

 sary by this singular mechanism, by which the 

 iiiTvr-; supplying the sense of touch may be 

 enabled to accommodate themselves to such 

 sudden and extensive changes of position ; for 

 this purpose the nerves supplying these organs 

 are of great length, reaching with facility to the 

 end of the tubes when protruded, and in their 

 retracted state the nerves are seen folded up 

 within the body in large convolutions. In the 

 figure, a a indicates the origins of the retractor 

 muscles of tlie foot from the columella; b, the 

 rinht Mipeiior tentacle fully protruded; r, the 

 left superior tentacle partially retracted ; d, the 

 left inferior tentacle extended, and e, the right 

 inferior tentacle fully retracted and concealed 

 within the body ; /', llic nerve supplying the 

 superior tentacle elongated by its extension ; 

 g, the retractor muscle of the same tentacle 

 arising from the common retractor muscle of 

 the foot and inserted into the extremity of the 

 tube ; h, the nerve of the opposite side thrown 

 into folds ; i, the retractor muscle of tli. 

 tentacle contracted ; k, the aperture through 

 which the nerve and retractor muscle enter the 

 tentacle rf; /, the brain; m, the suboesopliageal 

 ganglion ; n, the eye. 



/'/../cm. The eyes of Gasteropoda are ex- 

 tremely small in comparison with the bulk of 

 the animals, and seem more to represent the 

 rudiments of an organ of sight than to be 

 adapted to distinct vision. In many species 

 indeed they appear to be absolutely wanting, 

 en found, they resemble minute black points, 



Teniaclet and eye of Mure*. 



The natural size of the organ is seen in the 

 upper figure, in which on the right side the organ 

 is represented untouched, while on the left a 

 section has been made to exhibit its interior. 

 This section when magnified, as in the lower 

 figure, shews us that it consists of a spherical 

 cavity lined posteriorly with a dark choroidal 

 membrane, and containing a large spherical lens; 

 the position and structure of the retina we have 

 been unable satisfactorily to determine, although 

 the visual nerve may be readily traced to the 

 back of the choroid, where it seems to expand ; 

 but whether, as in the Cephalopods, its sentient 

 portion is spread out behind the pigment 

 which lines the eye-ball, or whether, as in the 

 forms of the organ common to the vertebrate 

 orders, the retina is placed anterior to the 

 choroid, is a question which we are at present 

 unable to solve. But however this may be, we 

 see anteriorly a distinct pupil surrounded by a 

 dark radiating zone, apparently an iris, to which 

 it corresponds at least in position, although 

 that it is really capable of contracting or en- 

 larging the pupillary aperture is more than our 

 observations warrant us in affirming. Finding, 

 therefore, the eye of the Murex to offer a struc- 

 ture which indubitably entitles it to be regarded 

 as an organ of sight, we are justified in consi- 

 dering the more minute specks of smaller Gas- 

 teropoda as similarly formed and subservient to 

 the same office. In the aquatic species the 

 eyes are generally placed at the base of the su- 

 perior or larger tentacles, although not unfre- 

 quently they are supported upon short pedicles 

 appropriated to them, as is the case in fhKotu 

 and others. In .!/;< r we have seen that the 

 tentacles which support them are large and 

 2 D 2 



