546 LECTURE XXII. 



The cutaneous muscular layer consists of oblique, longitudinal, 

 and transverse fibres, intimately united with the corium. Upon the 

 ventral surface it becomes very thick, and forms a long disc called 

 the " foot." The fibres of this part contract successively, so as to 

 form wrinkles or transverse waves following each other from behind 

 forwards ; whereby the disc glides over solid bodies or the surf^ice of 

 water. The circular foot of the limpet is used as an adhesive sucker. 

 In some species it expands to a great breadth ; in many it is ex- 

 tended lengthwise, and more or less cleft transversely; so that in 

 some species three divisions may be indicated : the anterior of these 

 is distinct in Natica j the middle division forms the chief creeping 

 disc ; the posterior one supports the operculum, when this is deve- 

 loped. The posterior lobe of the foot in the inoperculate Harpa is 

 said to separate spontaneously when the animal is irritated. In 

 Atalanta it is compressed, but supports the operculum : it seems 

 also to form the tail in the beautiful Carinaria, in which the 

 middle part of the foot is reduced to a small suctorial disc, sup- 

 ported by a part the form of a fin.* This mollusk and its allies, 

 hence called Heteropoda, swim on their back with their locomotive 

 foot upwards. The tentacles, buccal mass, and penis, have their 

 special retractor muscles in most Gastropods. 



In the living Cyprcea and Ovula the mantle-lobes are observed to 

 be in almost constant tremulous motion : but the most^ vigorous 

 muscular efforts in Gastropods are those of the foot combined with 

 the retractor shell-muscle. The Strombs and Scorpion-shells thereby | 

 progress by successive jumps ; the active Olives can turn over when 

 laid on their back, and bury themselves in the sand as the tide 

 retires ; the periwinkle advances alternately the sides of its longi- 

 tudinally indented foot ; Buccinum arcularia defends itself by its j 

 dentated operculum. 



At the grade of the Molluscous organisation which the Gastropods 

 have reached, their capabilities and spheres of action become more 

 extended and diversified than in the Pteropods and Acephala ; some 

 are terrestrial, some arboreal, whilst the more numerous aquatic 

 species are endowed with power to attain, subdue, and devour organ- 

 ised matter, dead and living. The nervous system of the Gastro- 

 pods is accordingly not only more complex and concentrated, not 

 only subordinated to better developed masses in connection with 

 organs of special sense and exploration, but it offers greater variety 

 in its general arrangement, and especially in the position of its gan- 

 glions, than in the Lamellibranchiate class, and with these modi- 

 fications considerable diffei-ences in the outward configuration of the 

 body are associated. 



* CCCVII. p. 198. 



