384 



GASTEROPODA. 



conchologist may derive important information 

 in examining mutilated remnants sometimes so 

 plentifully met with in calcareous strata. The 

 simpler shells (Patella, Fissurella) are formed 

 of very thin, compact, and parallel layers, 

 whilst in others three distinct strata of fibres, 

 each of which assumes a different direction, 

 may be observed. The fibres composing the 

 external layer are disposed perpendicularly to 

 the axis of the shell. In the middle stratum 

 the fibres are placed obliquely and are slightly 

 twisted, but so arranged that each meets at an 

 obtuse angle the extremity of one of the fibres 

 composing the outer layer, and in the internal 

 stratum they again assume a perpendicular 

 direction. Such a disposition of the fibres, 

 which is met with in all Siphonibranchiate 

 shells, is eminently calculated to resist ex- 

 ternal violence in whatever direction it may 

 act, and greatly contributes to the solidity of 

 the whole fabric. 



Operculum. Many of the spirivalve Gaste- 

 ropoda, especially such as are aquatic, are 

 provided with a calcareous plate, which is 

 placed upon the posterior surface of the body, 

 and closes accurately the mouth of the shell, 

 when the animal is retracted within it. The 

 texture of the operculum is sometimes horny, 

 but it is more frequently calcareous and of a 

 stony hardness, its contour being accurately 

 adapted to the orifice. It is composed of 

 parallel fibres disposed perpendicularly to the 

 base of the shell, and deposited in successive 

 layers around an axis, so as to give to the 

 whole structure the appearance of a solid 

 spirivalve, as may readily be seen on removing 

 it from the animal and examining its inner 

 surface. This has been looked upon by some 

 zoologists as analogous to the second valve of 

 bivalve Mollusca, to which, but for its want 

 of a ligamentous attachment, it certainly bears 

 a distant resemblance. 



The deciduous operculum of terrestrial 

 Gasteropoda, or epiphragma, as it is usually 

 called, is a widely different structure, being 

 merely an inspissated secretion, with which, 

 during the period of hybernation, the entrance 

 to the shell is closed ; and on removing the 

 outer plate, not unfrequently a second or even 

 a third similar membrane will be found within, 

 forming additional safeguards against intrusion 

 or the vicissitudes of temperature. 



During the progressive growth of the shell 

 the animal contained within it necessarily 

 changes its original position, advancing gra- 

 dually as the body enlarges from the earliest 

 formed spires towards the aperture, as may 

 easily be proved by sawing off the apex of a 

 spirivalve shell containing the living animal. 

 This circumstance is remarkably conspicuous 

 in some of the Bulimi (Bulimus decollatus), 

 enabling the occupant, as it grows, to break 

 off the turns of its spire which first contained 

 it, so that at the latter period of its life it does 

 not retain any part of its original shell. The 

 mode in which this advancement is effected is 

 a subject of much curiosity, as it involves a 

 power of detaching the muscles connecting 

 the creature with its abode, from the place 



where they were originally fixed, and forming 

 a new connexion with the shell ; but whether 

 this is effected by the removal of the original 

 fibres and the production of others more ante- 

 riorly, as is believed by some, or whether, as is 

 more probably the case, the creature has a 

 power of changing the attachment of its re- 

 tractor muscle at pleasure, is still a matter of 

 uncertainty. 



Organs of digestion. We shall not be sur- 

 prised to find that in a class so extensive, and 

 composed of individuals living in such diver- 

 sified circumstances, the alimentary organs are 

 much modified in form in different species, 

 according to the nature of the food with which 

 they are nourished. 



Mouth. In most instances the mouth pre- 

 sents the appearance of a retractile proboscis, 

 which can be protruded or shortened at the 

 will of the animal, but unprovided with jaws 

 or any apparatus for mastication ; it is in such 

 cases a muscular tube, formed of longitudinal 

 fibres prolonged from the common parietes of 

 the body, and of circular muscles, the former 

 serving for the retraction of the organ, the 

 latter causing its elongation by their successive 

 action; by means of this simple structure 

 every movement requisite for the prehension 

 of food is effected. At the bottom of the tube 

 is a narrow vertical aperture, the edges of which 

 are slightly cartilaginous, and behind this is 

 the tongue armed with spines variously dis- 

 posed ; the aliment therefore, having been 

 forced by the contractions of the proboscis 

 through the aperture at its termination, is re- 

 ceived by the tongue, and by the aid of the 

 latter organ is propelled into the oesophagus 

 without mastication or any preparatory change. 



In Buccinum and other syphoniferous ge- 

 nera, the structure of the proboscis is much 

 more complicated and curious, (jig. 182.) 

 " The proboscis, which carries with it the 

 oesophagus in its different states of protrusion, 

 is organised with wonderful artifice, being not 

 only capable of flexion in every direction com- 

 bined with limited power of retraction or 

 elongation, but it can be entirely lodged in the 

 interior of the body, folded within itself, so 

 that that half which is nearest the. base en- 

 closes the other portion : from this position it 

 is protruded by unfolding itself like the finger 

 of a glove or the tentacle of a snail, only it is 

 never completely inverted. We may repre- 

 sent it as composed of two flexible cylinders 

 (Jig. 182, a, 6,) one inclosed within the other, 

 the upper borders of which join, so that by 

 drawing outwards the inner cylinder, it is 

 elongated at the expense of the other, and 

 on the contrary, by pushing it back, the internal 

 cylinder becomes lengthened by its shortening. 

 These cylinders are acted upon by a number 

 of longitudinal muscles (c, c), all very much 

 divided at each extremity, the internal or su- 

 perior divisions being fixed to the parietes of 

 the body, whilst at the other end they are 

 attached to the inner wall of the internal tube 

 () of the proboscis, along its whole length, 

 extending even to its extremity ; their action is 

 obviously to draw the inner cylinder, and con- 



