8 



!• KID I IIIO)' (1KI,A.\I). 



M.-N. Kl. 



Jdhi the iiiai/^iiial iti lli which itjini ihc oiiUiiiujst rows (;/. th when counting 

 (rom number i ). 



Kach tooth consists of a basal plate ih. pi.) and the edge or epithem 

 (/'.) which, forming an acute angle with the first, is directed backwards — 

 in all figures, however, points upwards — and is covered by a transpaient 

 plate of somewhat different shape. The epithem of the central teeth has three 

 cusps: in the middle the mesoconus( m.-c.) and on each side the ectoconus; the 

 mesoconus as a rule has unconspicuous secondary cusps is. c). All the other 



teeth are unsymmetrical ; in the great majority 

 of cases there is an ectoconus {e.-c] lateral to 

 the mesoconus, and sometimes there may be 

 an endoconus medially to it. In fig. 3 there 

 do not appear certain minute structures which 

 are difficult to differentiate. However, they are 

 drawn on the figures of the radula in each 

 species. 



Close behind the buccal mass is con- 

 centrated the central nervous system, which 

 in the shape of a ring encloses the anterior part 

 of the oesophagus (fig. 4). The dorsal part is 

 formed by a pair of cerebral ganglia, whilst 

 ventrally there are situated two pairs of ganglia, 

 the pedal and visceral ones, each fused together 

 in the medial line and separated from the other 

 pair by a fissure. Lateralh' the nerve-ring 

 consists of a cerebro-pedal and a cerebro- 

 visceral connective on each side of the oesophagus. — From the central 

 ganglia, nerves proceed to various parts of the body ; in figure 2 the powerful 

 pedal nerves are to be Seen. These originate in the pedal mass and innervate 

 the foot-sole. — Between the pedal nerves we observe the long supra- 

 pedal gland \pcd. gl.) which in all stylommatophora produces the mucous 

 ribbon necessary for the locomotion of these animals (Kunkel 1903), and 

 which appears in the well-known mucous tracks where the animals have 

 crawled. Like a flattened tube, with a medial furrow on its upper side, 

 this gland lies upon the muscles of the foot-sole, opening with a transverse 

 aperture close below the mouth. In a transverse section of the gland 

 there may easily be seen numerous secretory cells and the ciliated parts 

 of the interior. 



Of the muscular system, we shall here only pay attention to the 

 columellar muscle, which, as previously stated, in the Arions is separated 

 into the components, viz. a pair of tentacular retractors (/. /. retr. and r. t. rctr.) 

 and a single pharyngeal one {ph. rctr.). Originating fi-om the roof of the 

 primary body-cavity close behind the kidney, each of the tentacular retrac- 

 tors passes like a broad ribbon to an eye-bearing tentacle, sending off a 



Fig. 4. Diagram of the central 



parts of the nervous system in 



Arion. 



cer. g. cerebral ganglia, ped. g. 



pedal ganglia, vise. g. visceral 



ganglia. 



