THE CARNIVOROUS SLUGS uF SOUTH AFRICA. 141 



larger, and the buccal retractors are more highly developed, 

 and it might therefore have been expected that these nerves 

 would have been larger and more conspicuous than in the 

 other species of Apera. The opposite is the case. In this 

 form the corresponding nerves are extremely slender. They 

 are joined for some distance with the cerebro-buccal connec- 

 tives (PL XIII, fig. 52), and then leave these and unite with 

 the sheath of the odontophore at the anterior end of that 

 organ. A possible explanation of this apparent anomaly will 

 be found in my account of the morphology of the buccal 

 retractors.^ 



The Buccal GtAnglia. — The buccal or stomato-gastric 

 ganglia are situated close together on the dorsal surface of 

 the odontophore, just behind and beneath the front end of 

 the oesophagus. In Apera burnupi and A. sexangula 

 they are usually in front of the cerebral ganglia, the cerebro- 

 buccal connectives curving forwards (PI. XIII, figs. 56 and 

 57). In A. dimidia and A. purcelli they are normally 

 situated a short distance behind the cerebral ganglia, when 

 the odontophore is not protruded (figs. 54, 55). Lastly, in 

 A. gibbonsi and A. parva the buccal ganglia are generally 

 very far behind the cerebral ganglia, and the cerebro-buccal 

 connectives are unusually long in consequence, often attaining 

 a length of 5 or 6 mm. in the former species (PI. XIII, figs. 

 52, 58, and PI. XV, fig. 70). This posterior position of the 

 buccal ganglia and the consequent lengthening of the 

 cerebro-buccal connectives is not uncommon among carni- 

 vorous snails and slugs, and is entirely due to the increase in 

 size of the buccal mass corresponding to the growth of the 

 radula. As the buccal mass becomes larger and longer, the 

 opening of the oesophagus, with the buccal ganglia behind it, 

 is pushed back until it comes to lie posterior to the cerebral 

 ganglia, and should this process be continued still further, 

 the cerebro-buccal connectives must be correspondingly 

 lengthened if the cerebral ganglia are to remain in their 

 original position near the sense-organs. Moreover, the 

 ' See pp. 17.3. 174. 



