l6o ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



interval from the cornea, so that there is hardly any anterior chamber 

 of the eye. There is no iris, but the inner surface of the posterior 

 chamber is coated by a layer of dark chocolate-coloured pigment.^ 



The Auditory Vesicles (J), fig. 7. — These lie behind and a little 

 below the cephalic ganglia. Each is a spherical vesicle, about -yJo-dth 

 of an inch in diameter. Its walls are irregularly thickened here and 

 there, and it contains a spherical otolithe of about half its diameter. 

 I was unable to perceive any motion in the otolithe. The auditor}' 

 nerve is a delicate thread arising from the under surface of the supra- 

 cesophageal ganglion, just in front of the commissural cord. It appears 

 to terminate suddenly on entering the vesicle. 



This origin of the auditory nerves from the cephalic ganglia, when 

 the pedal ganglia are zvell marked and placed beloiv the oesophagus, is a 

 circumstance common to all the Heteropoda,^ and, so far as I am 

 aware, altogether peculiar to them among the Mollusca. The only 

 writers who appear to have been struck by it are MM. Frey and 

 Leuckart : they say that the auditory organs are united with the 

 supra-oesophageal ganglia " only when the lower oesophageal ganglia 

 are ■\\'anting (except in Carinaria, in which the great length of the 

 lateral commissures of the oesophageal ring appears to have made such 

 a position necessary)." (Beitrage, p. 55.) I must confess I do not 

 see the force of this explanation ; and the lower oesophageal ganglia 

 are never " wanting," though they may be united with the upper ones. 



II. Anatomy of Atlanta. (Plate III. [Plate 18]) 



This is a very small and very beautiful pelagic mollusk, with a 

 .shell not more than one-fourth of an inch in diameter. It appears 

 to be identical with the Atlanta Lesuerii of Eydoux and Souleyet. 

 Its structure resembles that of Firoloides in all its essential points, 

 and the transition between the latter and Atlanta is complete through 

 such forms as Firola, Carinaroides, and Carinaria. 



The shell is flattened and spiral, none of the whorls projecting 

 be)'ond the plane of the outermost. The aperture is notched on its 



' I do not see that the eyes of Heteropoda are so " peculiarly formed" as Krohn has it 

 ( Ffrner Beitrag zur Kenntniss des Schneckenauges. MiiUer's Avchiv, 1S39). What I have 

 described as the true cornea, is by Krohn considered as an anterior portion of the vitreous 

 humour ; such an arrangement would of course be vei-y peculiar, but I think that my account 

 is correct. The eye, it would appear, projects much less in Carinaria and Plerotrac/iea ; and 

 in the latter, according to Krohn, there is even a rudimentary eyelid. 



Krohn does not say anything about the muscles of the eye in these two genera. 



= Compare ]\Iilne-Edwards, Sur divers Mollusques, Annales des Sciences, 1842, and the 

 figures of Eydoux and Souleyet, so often referred to. 



