408 



THE POPULAE EDUCATOR 



ganglia of the body are in direct communication with the 

 cephalic pair, but not with the other two pairs. We may there- 

 fore consider that the cephalic pair is the originator of the 

 voluntary actions, or is concerned in all those actions which 

 the creature performs as a whole and individual. This pair is, 

 BO to speak, the central telegraph-station, and therefore may be 

 considered to be 

 in direct commu- 

 nication with the 

 manager. 



We have dwelt 

 BO long on the 

 nervous system 

 of the conchif er, 

 partly because we 

 had not space for 

 it in our last les- 

 son, and partly 

 to indicate the 

 contrast which it 

 presents to that 

 of the gastero- 

 pods or headed 

 molluscs. In 

 these, though the 

 nervous system 

 no doubt consists 

 of the same ele- 

 ments, they are 

 more closely as- 

 sociated ; and the 

 ganglia, situated 

 over the throat, 

 behind the cavity 

 of the mouth, are 

 in direct commu- 

 nication with the 

 organs of sense ; 

 and this is the 

 same arrange- 

 ment as is found 

 both in verte- 

 brates and arti- 

 culates. 



The Gastero- 

 poda derive then- 

 name from the 

 usual form of the 

 locomotive or- 

 gan, which is so 

 constantly found, 

 though so vari- 

 ously developed, 

 in the different 

 members of this 

 class. We found 

 the foot in the 

 Conchifera to be 

 an organ which, 

 in some, secreted 

 the byssua or 

 anchor-cable, in 

 others bored 

 holes, and yet 

 in others accom- 

 plished jerky 

 movements of 

 the body. In the 



swan-mussel of our rivers this instrument is applied to more 

 regular and definite locomotion, and with the foot they may be 

 seen ploughing their way through the soft mud which falls to 

 the bottom of the stream. In their case, however, the foot is 

 a rounded organ, and at its end is something like the human 

 tongue, both in shape and structure. In the gasteropods, or 



II. 



12 



PULMOGASTEROPODA. 1. ARION (THE BLACK SLUG). II. AGATHINA MAURITANICA. III. CYCLOSTOMA 



ELEOANS. IV. DIAGRAM OF THE CIRCULATION IN A SNAIL. 



Kefs, to Nos. in Pigs. I. 1, orifice of lung-chamber ; 2, anus. II. 1, throat ; 2, stomach ; 3, 

 intestines; 4, anus ; 5, liver; 6, 6, pulmonary diaphragm ; 7, main vein ; 8, chamber surrounding 

 the heart ; 9, auricle (receiver) ; 10, ventricle (distributor) ; 11, kidney ; 12, generative organs. 

 (N.B. In this diagram the shell has been removed and the skin cut along the back and opened ; 

 the floor of the lung also is thrown aside.) III. 1, operculum. IV. 1, lung vein; 2, auricle; 

 3, ventricle ; 4, main artery ; 5, liver artery ; 6, foot artery ; 7, stomach artery ; 8, buccal 

 cavity ; 9, salivary gland. 



teropods ; but in function, of course, the foot of the gasteropoda 

 is much more like a foot than the same organ in the lower class. 

 Usually the foot is a muscular, elongated sheet, broader and 

 longer than the body of the animal, and acts at the same 

 time as the wall of the body and the means of propelling it 

 along. The whole rim of the foot all the way round is usually 



thickened, and 

 can be closely ap- 

 plied to a smooth 

 surface, while the 

 central parts can 

 be thrown in 

 wrinkles. Thus 

 the whole acts as 

 a kind of sucker 

 or holdfast, while 

 all the middle 

 parts, being al- 

 ternately applied 

 to the ground 

 and dragged over 

 it, effect a move- 

 ment in which 

 the whole animal 

 participates. If 

 the reader allows 

 a slug to crawl 

 up a pane of 

 glass, and looks 

 at it through the 

 transparent me- 

 dium, he will see 

 successive waves 

 moving all along 

 the foot, show- 

 ing that, while a 

 series of points 

 are fixed, the 

 parts in between 

 are moving, and 

 the moving parts 

 then become 

 fixed, allowing 

 the previously 

 fixed parts to be 

 pushed or pulled 

 along by the con 

 traction of the 

 muscles embed- 

 ded in the skin. 

 Such a mode 

 of progression, 

 which may be 

 called piecemeal, 

 is, of course, 

 very alow, but 

 it is sure; and 

 how should an 

 animal without 

 limbs move over 

 a solid surface 

 otherwise ? As- 

 sociated with this 

 power of definite 

 locomotion, slow 

 as it is, the whole 

 organismis modi- 

 fied. 



Let us suppose that a Lamellibranch had the under part of 

 its foot flattened into a broad muscular sheet, capable, not of 

 pushing through soft mud, but of gliding over smooth rock : 

 how could it make use of its new power of locomotion ? It 

 would, in the first place, be hampered with two immense shields, 

 which, being ample enough to close upon its whole body, would 



belly-walkers, the foot is a flat broad surface placed along the ] certainly have their edges dragged over and ground upon the 

 under side of the body, by means of which the animal can cravd. rock over which it passed, and thus wrenched about in relation 

 over solid bodies. In some of the conchif ers the shape of the foot to one another and to the soft parts of the animal united to them. 

 is much more like that of the human foot than in any of the gas- I Then its large sheets of unprotected membrane, called gills, 



