44 Mr. C. Spence Bate on the Homologies of the Carapace 



impressions of sound, enclosed as it is within and covered by a 

 stout calcareous operculum. 



But if we view it as an organ of smell, every objection pre- 

 viously becomes evidence in favour of the idea. The small door 

 when it is raised exposes the orifice in a direction pointing to the 

 mouth ; this also is the direction of the same organ in all the 

 higher orders. In Amphipoda it is directed inwards and for- 

 wards. In every animal it is so situated that it is impossible for 

 any food to be conveyed into the mouth without passing under 

 this organ, and of this the animal has the power to judge its 

 suitability for food by raising the operculum at will, and expo- 

 sing to it the hidden organ — the olfactory. 



If we turn to the upper antenna, we find that its position, 

 form and power are as capable of fulfilling the office of conducting 

 the sensation of sound, as the lower is that of smell. As I before 

 observed, it is always placed erect, and continually feeling in the 

 water for the first approximation of sensation. The filamentary 

 appendages are always two or more, one of which is supplied 

 with singularly delicate membranous cilia, being apparently pro- 

 longations of a similar membrane to that which covers the larger 

 orifice of the olfactory organ. These lengthened and delicate 

 cilia are peculiarly adapted to receive and convey the most minute 

 vibratory sensation of the medium in which they are suspended. 

 These organs when spoken of may conveniently be designated 

 as auditory cilia, and have been found in every species of Crus- 

 tacea that has been searched for them. 



If we turn our attention to the internal structure of this an- 

 tenna, we shall find that it supports the idea of its being an 

 auditory organ more forcibly than its external analysis. 



In the Brachijura, as before observed, the first or basal arti- 

 culation is largely developed ; if it be removed from its connexion 

 with the animal and broken open, the basal articulation will 

 be found occupied by a still smaller chamber having calcareous 

 walls of a nuich more delicate character than the integumentary 

 structure. This internal chamber or cell is that which in this 

 paper is supposed to be a cochlea, from its analogy both in its 

 structure and supposed use to that organ in higher animals. It 

 is situated in the cavity of the basal articulation of the antenna, 

 and attached to the walls furthest from the median line of the 

 Crab. It presents a tendency to a spiral form, but passes not 

 beyond the limits of a single convolution. The calcareous walls 

 extend across the axis of the ideal spire, and the internal cavity 

 is one continuous irregular chamber, the walls of which at the 

 centre of the axis closely approximate so as almost to meet. 

 PI. II. figs. 3, 4, 5, 6. 



