12 MR MACDONALD ON THE ANATOMY 



is distinctly apparent. In Firoloides, Firola, Cardiajjoda, and Carinaria, the 

 auditory nerves are of considerable length, so that the ear-sacs are, as it were, 

 appended to them ; but in Atlanta, which embraces more of the characters of the 

 true gasteropods, those sacs are closely applied to their proper ganglia. 



The gullet and anterior or buccal division of the aorta pass through a nervous 

 collar consisting of four supra (Plate II. fig; 1. f), and two sub-oesophageal (Plate 

 II. fig. 1, i) (pedal) ganglia, connected by two lateral commissural chords, each of 

 which arises from the two superior ganglia of the corresponding side by distinct 

 slips. The two anterior cerebroids supply the short but stout optic nerves to the 

 eyes, while the acoustic sacs lie in the outer angle between these and the pos- 

 terior ganglia, having no connection whatever with the sub-oesophageal or pedal 

 nervous bodies. 



The two principal nerves supplied to the buccal mass and mouth arise from 

 the anterior superior ganglia, pass forward on either side of the buccal artery, 

 and at some little distance behind the lingual cartilages divide into a superior 

 and inferior branch, and thus, communicating also with the buccal ganglia, are 

 equally distributed to the muscular and other structures in this locality. 



The pedal ganglia give off branches to the several parts of the foot, and to the 

 muscular sheath of the body, and two stout commissural nerve-trunks arise from 

 their posterior border and join the visceral ganglia, which preside over the heart's 

 action and the functions of the neighbouring organs and parts. As there is much 

 difficulty in tracing out the filaments derived from those ganglia in Atlanta, their 

 homologues in Firoloides may be studied with advantage. 



Digestive System. — Although the cylindroid proboscis exhibits great pliancy in 

 itself, the range of its movements is nmch augmented by the mobility of the 

 neck. Destitute of labial teeth, the lining membrane of the mouth is prolonged 

 into the fauces and oesophagus above and behind, being continuous with the lining 

 of the tongue-sac inferiorly, blending with the borders of the dental ribbon, which 

 doubles over the fore-part of the supporting cartilages (Plate II. fig. 1, h), and thus 

 projects into the oral cavity. The cartilages just noticed form the basis of the 

 tongue, and consist of two principal pieces of an oblong figure, forming, by their 

 union along the mesial line, a grooved pully-like surface, over which the lingual 

 strap glides. In many gasteropods, a smaller piece of cartilage is articulated to 

 the posterior extremity of each of the principal ones, so as to admit of greater 

 mobility, and a certain amount of compression in the longitudinal direction; and 

 I have reason to believe that corresponding pieces exist in the framework of the 

 tongue of Atlanta. The cartilages are connected together and acted upon by trans- 

 verse and oblique slips of muscular fibres, and small bundles pass from the inner 

 surface of the tegumentary covering, to be inserted into them at different points. 



The lingual strap, commencing by a small point beneath the anterior extremity 

 of the basal cartilages, at first passes a little forwards, then turns upwards and 



