26 



muscle of adhesion sometimes encircles the back in the form of a horse-shoe, 

 as in Sijihonaria, wliilst in the cup-and-saucer Limpets {Cahjptraa) it is 

 attached to the cup-shaped appendage by wliich that group is characterized. 

 In the spiral Gastropods the shell is connected by a thin riband-like muscle 

 wliich has its point of attachment on the axial pillar or columella ; and it 

 is by the elasticity of tliis muscle that the animal advances its head and 

 foot and again retires within the last whorl. There are, however, instances 

 in wliich the spiral Gastropod, desirous of evacuating a portion of its shell, 

 has the power of sliding the attachment of the columellar muscle, without 

 relaxing it, independent of the requirements of its increase of growth, in a 

 manner similar to the periodical slipping of the muscular girdle in the 

 growing Nautilus. In the Helix decoUatus and many species of Tupa, the 

 early portion of the shell is allowed to fall away, and the whorls of occupation 

 are roofed in by a new concentric layer; in many species of Melania, and 

 other inhabitants of fresh and stagnant water, the apical extremity of the 

 shell becomes eroded and consumed ; whilst that singular tenant of the 

 coral rock, the Magilus, fills up the evacuated portion of its shell, to a consi- 

 derable extent, with solid testaceous matter ; an operation which would pro- 

 duce an incumbrance incompatible with the locomotive faculties of a free 

 agent. There is also an additional muscle in those species which are oper- 

 culated for the purpose of drawing the operculum within the aperture. 



The operculum is a shelly or horny plate adapted in most species to close 

 over the soft parts when retracted within the shell ; it is, however, often re- 

 presented by merely a small tliin horny plate, in no wise fitting the aperture 

 of the shell, and not apparently answering any purpose in the life and eco- 

 nomy of the moUusk. The variations in the growth and substance of the 

 operculum, moreover, afford characters of very secondary importance ; as we 

 find it sometimes shelly, and sometimes horny, in the same natural group. 

 The genus TrocJms, for example, offers some species with the former, some 

 with the latter ; and among the Copies and Volutes the operculum, though a 

 rudimentary one, is present in some species and not in others. 



The Gastropods evince a very low degree of sensibility ; in many species 

 the tentacles, and with the tentacles the eyes, and even some parts of the 

 head are reproduced after amputation, provided there is no displacement of 

 the cerebral ganglions.^ Some are also capable of suspending aU signs of 

 vitality for a very considerable period, I remember an instance of some 

 Helices having been received from a distant locality and kept in a dry 

 lumber box for two years in a state of torpor, from which they were fully 

 released upon being placed on a moist fresh leaf. All the Gastropods ex- 

 hibit great skill in the repair of any injuries done to their shell, and consi- 

 derable economy is exercised in absorbing or smoothing down any spines or 



* Baron Fenissac slates that he has seen the terrestrial Gastropods or Slugs allow their skins 

 to be eaten by others, and in spite of large wounds thus produced show no sign of pain. — Owen. 



