46 On the Function of the Antenna in Crustacea. 



Fig. 3. The anterior aspect of the internal or first antennal ring isolated 

 from the rest of the carapace, answering to A A A in fig. 2, 

 which is the internal aspect of the same. 

 a. The line of demarcation between the dorsal and ventral arches, 

 which enlarges at (e) to permit the organ of vision to pass 

 through. 

 h, b. Portions of the orbits. 



c. The superior antenna in situ. 



f. The foramen through which the nerves and muscles pass to the 

 superior or internal antenna, which is here removed. 



d, d. Depressions left by the removal of the external antenna, the 



extremity of the third joint of the peduncle of which here rests, 



with which it is almost fused. 

 Fig. 4. Carapace of Diastylis (Say) Rathkii, Kroyer, = Alauna rostrata, 



Goodsir, Bell : dorsal aspect. 

 F^g. 5. Ditto : ventral aspect. 

 Fig. 6. Lateral view. 



Fig. 7. Carapace of Cancer Pagurus : pupa stage ; ventral aspect. 

 Fig. 8. Carapace oi Astacus jiuviatilis : dorsal aspect. 

 Fig. 9. Do. do. do. : lateral view. 



Fig. 10. Carapace of Corystes Cassivelaunus : lateral view. 



Plate II. 



F^g. 1 . Carapace of Cancer Pagurus : ventral aspect. 



Fig. 2. Cephalic ring of Erichthonius difformis, to show the relation in 

 which it stands to the carapace of Podophthalmia. The antennal 

 rings, which in the Sessile-eyed order are absorbed within the 

 posterior, are here produced in dotted lines (a)*. The letters in 

 all define the homological divisions. 



a. The two antennal rings. 



b. The mandibular. 



The line between a and b is the limit of separation between the 

 rings and homologies in Brachyura with the cervical suture 

 in Macroura. 

 Fig. 3. The anterior or internal antenna of Cancer Pagurus. The dotted 



line at (a) shows the position of the internal cell. 

 Fig. 4. Posterior view of the basal articulation of the antenna, showing the 

 internal or auditory cell (the presumed cochlea of the author). 

 Fig. 5. Anterior view of the internal cell ; the outer or dermal walls of 



the antenna being broken away. 

 Fig. 6. Internal view of the cell, which is broken to show the relative ar- 

 rangements of its waUs. 



* The author would wish this passage to be accepted only provisionally 

 for the present, as, since the paper has been in the hands of the printer, 

 on examining some specimens of Gammarus which had been saturated in 

 dilute acid, he thinks he has identified a line or suture which must 

 homologize with the cervical suture in the higher types. Should further 

 research establish the fact, then it must result that the inferior antennal, 

 as well as the mandibular segment, enters into the structure of the first or 

 cephalic ring of the Amphipoda ; but this he hopes to discuss soon more at 

 large elsewhere. 



