THE CARNIVOROUS SLUGS OP SOUTH AFRICA. 15-'^ 



cerebral division, which is connected with the sense-organs of 

 the head/ and is the only section in which the nerve-cells are 

 aggregated to form ganglia on the dorsal side of the alimentary 

 canal. Thirdly, there is the pedal section, which innervates 

 the foot and pedal gland. Fourthly, there is the pleural 

 section, innervating the sides of the body and head. And 

 fifthly, there is the visceral section, which innervates the 

 neighbourhood of the mantle-cavity. 



And just as there are five pairs of ganglia, so also are there 

 five commissures ventral to the alimentary canal, as will be 

 clearly seen from the diagram. Now some authors have 

 thought that there were only three such commissures in the 

 P u 1 m o n a t a, and very few have found more than four. Must 

 we then regard Apera as exceptional in this respect? I 

 think not. For it seems probable that further researches will 

 show that there are normally five commissures ventral to the 

 alimentary canal in the P^uthyneura, although the slender 

 sub-cerebral commissure may have entirely disappeared in 

 some Pulraonates, just as in the Strep toneura the corre- 

 sponding labial commissure seems to have disappeared in many 

 of the Pectinibranchia. 



THE STEUCTURE OF THE UPPER TENTACLES, AND 

 THE TENTACULAR RETRACTORS. 



In PI. XVI, the figs. 80-86 represent photomicrographs 

 of serial sections through the left upper tentacle of Apera 

 dimidia in its retracted condition. It will be seen that 

 the olfactory organ is large and extends forwards in front 

 of the eye, thus reducing the width of the cavity in the 

 retracted tentacle. The eye itself appears to be of the 

 usual structure, the small size of the lens being possibly due 

 to contraction. The retina is very deeply pigmented. The 



^ It is not improl)able that the motor nerve to the buccal retractor 

 should be regarded as belonging to the pleural section, for it arises 

 close to the origin of the cerebro-plenral connective. 



