224 HUGH WATSON. 



Agnatlia. If we find that two agnathous families only 

 resemble each other in those features which are likely to 

 have been acquired through carnivorous habits^ and are very 

 similar to diiferent gnathophorous families in their other 

 characters, then we may assume that they are probably of 

 different origin ; but if we find that the resemblances between 

 two families of agnathous snails cannot all be explained in 

 this way, and that these families are in their most important 

 features more similar to each other than to any families of 

 the Gnathophora, the probability is that they are closely 

 related. It is evident, however, that before we can discuss 

 the affinities of any particular genus, we must have a clear 

 idea as to how a snail is likely to become modified if it adopts 

 carnivorous habits, and which of the features characteristic of 

 Apera, Testacella, and the other genera included in the 

 Agnatha, are likely to be due to their animal food. 



Snails and slugs find their food chiefly by means of their 

 sense of smell, and one might expect this sense to be especi- 

 ally well developed in the carnivorous forms, because animals 

 which move slowly enough for snails to catch them must be 

 very much more difficult to find than plants. Now in man}^ 

 of the Agnatha, and more especially in the Rhytididas 

 and the 01eacinid{«, the olfactory organs at the tips of the 

 upper tentacles are so large that the eye comes to occupy a 

 position some distance behind the extremity.^ Moreover 

 Plate has shown that Testacella, unlike most of the 

 Stylommatophora, retains a pallial olfactory organ. The 

 unusual development of these sense-organs might be expected 

 to lead to a corresponding development and concentration of 

 the sensory nerve-centres, and accordingly we find that in 

 nearly all the carnivorous forms the cerebral ganglia are 

 large and close together, Phrixolestes being perhaps the 

 most notable exception. 



Carnivorous snails and slugs prey chiefly upon the herbi- 



' See Strebel, H., 'Beitrag z. Keiintn. d. Fauna Mexikan. L.- u. Siiss- 

 wasser-Conchyl.,' 1878, vol. iii, pi. xv, fig. 1 c; Suter, H., ' Jourii. of Mai.,' 

 1899, vol. vii, pi. iii, fig. 1 a. 



