THE CARNIVOROUS SLUGS OF SOUTH AFRICA. 137 



the Trigone chl am inte/ the cerebral ganglia are situated 

 very near together on the dorsal side of the buccal mass. 

 Indeed, in Apera gibbon si they are so closely bound to 

 each other by connective tissue that they appear as one 

 (PI. XIII, fig. 52) ; and it is only in the smaller species, such 

 as A. parva (fig. 53) and A. purcelli (fig. 55), that they 

 can be clearly seen to be separate ganglia without the help 

 of any reagents. When fi*eed from their surrounding tissue, 

 the ganglia are seen to be oval structures, nearly twice as long 

 as they are broad, and rather nearer together behind than in 

 front (PI. XV, figs. 70-72). Their average length in Apera 

 is about f mm. Towards the outer side of the front end of 

 each ganglion there is a slight eminence formed by the 

 accessory lobe. This is best seen in A. dimidia. In this 

 species the apex of each lobe is prolonged into a filament, 

 which is very like a slender nerve (fig. 71), but is probably 

 the remains of the embryonic cerebral tube which originally 

 connected the accessory lobe with the exterior. Similar 

 accessoiy lobes have been figured by de Lacaze-Duthiers ~ in 

 Testacella, l)y Wiegmann'^ in Ennea (Edentulina), and 

 by Beutler ^ in Paryphanta, and the last author has 

 described them at some length. 



The cerebral ganglia are joined by two commissures, 

 namely, the cerebral commissure above the buccal mass, and 

 the sub- cerebral commissure below it. The cerebral com- 

 missure is very short and broad, and is situated somewhat 

 towards the posterior ends of the ganglia (PI. XV, fig. 71). 

 The sub-cerebral commissure, on the other hand, is very long 

 and extremely slender. It arises from about the middle of 

 the outer sides of the ganglia, and encircles the buccal mass 

 just in front of the cerebro-pedal connectives and the pedal 

 ganglia, being closely bound to these by connective tissue 



' Simrotli, H., ' Festschrift Leuckarts,' 1892, pp. 53, 55. 

 ' 'Arch. Zool. Exper.' (2nd ser.), 1887, vol. v, pi. xxxviii, figs. 70, 71. 

 3 ' Mitt. Zool. Samml. Mns. Nat. Berlin,' 1898, vol. i, pi. iii, fig. 7. 

 * ' Zool. Jahvb..' 1901, vol. xiv, pi. xxix, figs. 58, 61. and pp. 400-402 ; 

 see also Pelseneer, • Mem. Acad. Roy. Belg.,' 1901, ex. vol. liv, pp. 31-37. 



