AI^D CLASSIFICATION OF THE HETEROPODA. 3 



of much importance in distinguishing the Heterojyoda. In Cardiapoda and Cari- 

 naria, as well as in the naked genera, the acoustic sacs are, as it were, appended 

 to the auditory nerves, which are of considerable length, and arise from the supra- 

 oesophageal ganglia. In the latter particular the heteropods differ from most of 

 the true gasteropods, in which the special centre of audition is incorporated with 

 the suboesophageal ganglia. 



The oral aperture is circular, and at the extremity of a lengthy muzzle, within 

 which the " buccal," or rather the lingual mass, appears to occupy but a small 

 space. There are no labial or maxillary dental organs, but the lingual armature 

 is highly characteristic of the order. In accordance with the laws of its develop- 

 ment, the lingual ribbon gradually increases in breadth from before backwards. 

 The rachis consists of a single series of plates, while there are three members in 

 each pleura ; and all the dental points are simple, sharp, and conical, either per- 

 fectly straight and projecting backwards, or slightly curved and inclined more or 

 less inwards. 



The segments of the rachis bear a variable number of simple teeth ; but the 

 internal pleural plates, whose attached surface occupies nearly, or quite the whole 

 breadth of the pleurae, generally present one large dental process at the inner 

 extremity, with a much smaller one somewhere near its base. The absence of 

 this smaller tooth, or, when present, its internal or external position with respect 

 to the larger one, may be taken into account in classification ; but as the two 

 outer members of the pleurae in all the Heteropoda are in the form of simple claw- 

 shaped uncini, any specific characters afforded by them can only be of a relative 

 nature, as to proportionate length, amount of curvature, thickness, &c. On ex- 

 hibiting my preparations and drawings to my respected friend, Mr W. S. Macleay, 

 he saw very plainly that generic characters might be drawn from the rachis alone ; 

 and it was with no small satisfaction that I enjoyed the concurrence of so great a 

 man in my first attempt to effect a classification of the Heteropoda, by their 

 lingual dentition, which so often outlives the decay of the soft parts generally. 



The gills, or branchiae, usually consist of a linear series of short claviform or 

 tapering processes, with a loose cellular investment, presenting a longitudinal 

 zigzag fold on the inner surface, giving rise to the plumose appearance sometimes 

 so incorrectly represented in figures of these animals. In Firoloides^ however, 

 I have never seen any vascular appendages of the nature of gills, and they are 

 frequently absent, or inconspicuous even in Atlanta. 



In closing this general sketch, it only remains to be stated, that the sexes are 

 distinct in Heteropoda, as was first suspected by M. Laueillard, who assisted 

 the great Cuvier in his dissections. Mr Macleay further informed me that 

 CuviER, on the same account, was deterred from placing them amongst his Tecti- 

 hrandiiata. Modern anatomists, however, have thought fit to contradict this 

 opinion, though upon what grounds, except hearsay, it is difficult to determine. 



