170 HUGH WATSON. 



differences between the structure of the odontophore in 

 Apera and Natalina are of little morphological impor- 

 tance, and are certainly less striking than the differences 

 between the raduljB of the two genera, for the radulse of 

 Natalina and Apera have become highly specialised in 

 different directions. 



Turning now to the TestacellidfC; we find slugs which 

 are vermivorous like Apera gibbon si and have radulas 

 remarkably similar to the var . lupata of that species. Yet an 

 examination of the odontophore ofTestacella maugei Fer. 

 reveals many features in which the arrangement of its muscles 

 is quite different from anything that we have found in Apera 

 or Natalina (PL XXIV, figs. 155, 156). The structure of the 

 odontophore ofTestacella maugei differs from that which 

 I have described as characteristic of Apera in the following 

 respects. — (1) There is no terminal retractor (unless it is 

 represented by the small connections between the floor of the 

 radula-sac and the median retractor). (2) Perhaps in conse- 

 quence of this the radula-sac is shorter than the ventral 

 pocket, instead of being much longer. (3) The median dorsal 

 muscle is short, very oblique, and divided into three or four 

 line strands. (4) The median retractors curve together above 

 the radula-sac to form a tube which is continued to the hind 

 end of the odontophore. (5) The muscles which form the top 

 of this tube are not only firmly attached to the lateral retractors 

 on each side, but also to the sheath of the odontophore ; and 

 they are continuous with the extrinsic buccal retractors, which 

 join the posterior half of the odontophore on each side of the 

 mid-dorsal line. (6) The lateral retractors are scarcely 

 divided into separate strands. (7) There seem to be no flexor 

 muscles along the upper edges of the support, but on its inner 

 surface there is a slight median longitudinal ridge which dis- 

 appears towards the hind end and is connected with the 

 median retractors by a few slender strands. (8) The ventral 

 muscles are rather stouter than in Apera and more definitely 

 split up into separate strands. (9) Anterior to these the 

 suspensor muscles are free, but directed obliquely downwards 



