272 TROCEKDINGS OF THE MALVCOLOGICAL SOCIETY. 



The reason for this great dovelopmont of the buccal mass is to be 

 found in the carnivorous habit of the nicmbers of the group. This 

 habit, in whatever chiss of animals it occurs, is invariably accompanied 

 by an enhxrgement, either of the prehensile organs, or of the jaw 

 armature (usually the teeth), or of both. In the case under considera- 

 tion, the buccal mass, with its odontophore, has to serve both as 

 a prehensile and a masticatory organ, and is consequently corre- 

 spondingly enlarged. 



NaUdina itself preys on snails, into whose shells it creeps, and then, 

 by the aid of its powerful buccal muscles and large hooked teeth, 

 literally scrapes the inhabitants out mouthful by moutliful. The 

 specimens kept alive by mo would clear out the sbcll of a large Helix 

 in a few days, and apparently kept steadily at work until this was 

 accomplished, rarely leaving anything but the kidney behind. The 

 greatest difficulty to be overcome by the cannibal must be the piercing 

 of the muscular foot of the victim, which structure when contracted 

 is extremely hard and compact, but this must nevertheless bo 

 penetrated in order to reach the underlying soft tissues ; and it 

 seems probable that the enormous muscular development of tlie 

 buccal mass is adapted for this end. since in the allied fonu 

 TestaceUa we find a very different modification of the muscles of the 

 buccal mass in connection with its different habit, viz. that of seizing 

 and holding the soft body of a worm, while its juices were being 

 sucked out by the muscular oesophagus of the slug. 



Musculature of the buccal mass. — Externally we find a thin layer of 

 longitudinal muscular fibres, which attains its greatest development 

 on the ventral side (PL XVII, Figs. 4, 5, 1 e.l.m.), the contractions of 

 this ventral muscle often causing a flexure on that side of the free 

 end of the buccal mass.' 



Beneath this is the great constrictor muscle, the fibres of which 

 are arranged transversely. This muscle is broken up into two well- 

 marked portions, especially posteriorly, viz. a small dorsal band 

 {(I.e., Fig. 5) and a more extensive and thicker ventral portion {v c.. 

 Figs. 5 arid 7); the fonuer is attached in places to the odontophoral 

 cartilage. 



The remaining muscles are modifications of an original simple 

 longitudinal band, now broken up by the intervention of the odonto- 

 ])lioral cartilage into an antero-ventral and a postero-dorsal series, the 

 latter having the greater development. 



The antero-ventral muscle supplies the motive force, which pulls the 

 radula forwards over the cartilage and erects the teeth ; tliis muscle 

 attains but slight development in Natalina {v. I., Figs. 5 and 7). It is 

 attached in front to the subradular membrane and behind to the 

 odontophoral cartilage ; externally it is continuous with a series of 

 tendonous muscles which are attached to the sides of the fuiu-tional 

 portion of the radula (Fig. 7), and which appear to be concerned in 

 flattening out that structure and separating the teeth. 



* A similar condition is fij^ured by Godwin- Austen for a contracted specimen of 

 rariiphnnta : Troc. Malac. Soc, Vol. I, 1893, p. 5. 



