ON OUR rUESENT KNOWLEDGE Ol' THE CRUSTACEA. 53 



to tlic extensive membrane that protects the internal viscera from the intro- 

 clTiction of the water. This membrane is continuous with and attached to 

 the inner surface of the posterior margin, and is represented generally by a lobe 

 that runs parallel with the posterior margin. This portion may conveniently 

 bo known as the postcardiac region. 



The hepatic regions extend on either side from the orbital region anteriorly 

 to the posterior tooth of the hepatic crest, and are bounded by the gastric and 

 branchial regions. This is a larger portion than is admitted by Milne-Ed- 

 wards, but it is one that corresponds with the extent of the hepatic viscera. 



The branchial region reaches from the posterior tooth of the hepatic crest 

 to the posterior margin, along which it traverses nearly to the median line 

 on either side, and is bounded on the inner side by the cardiac and genital 

 regions, and anteriorly by the hepatic regions, from which internally it is 

 separated by a thin membranous partition. 



These several divisions appear to me to be based upon strictly anatomical 

 grounds, and as such may be regarded as natural divisions, the variation 

 of which must depend upon that of structure, and therefore may be relied 

 upon as affording characteristic distinctions. 



The great consolidation of the anterior somites of the skeleton has led Prof. 

 Dana to pronounce the centralization to amount to a cephalization of the forces ; 

 but this opens a subject of considerable extent and interest, which, if permitted, 

 I hope to present in a continuation of this Eeport at the next Meeting of the 

 Association. 



EXPLANATION OP THE PLATES. 



References in each Plate the same :— C, Cephalic region ; 0, Orbital region ; 8 S, Sto- 

 niaohic region; P, Pyloric region ; iJi?; Hepatic region ; Gtl Gi;/,Gemtiil region; Car, Car- 

 diac region; Post-Car, Post-Cardiac region; M, Muscles connecting the pereion with the 

 pleon. 



Plate I. 



Fig. 1. Carapace oi Navj^lius, or earliest larval form of Crustacea. 



2. Carapace of Zoca, or second larval form. 



3. Carapace of Mcgaloim, or third larval form. 



4. Carapace of Diasft/lis. 



5. Carapace of Trilohita, with that of M'galopa displayed on it, to demonstrate the 



homological relation of the fissure on the ventral surface of the latter with that} 

 on the dorsal surface of the former. 



6. Carapace of Cancer j^affurus. 



7. First or ophthabuic somite of Cancer, with ophthalmic appendages and eyes 



attached. 



8. Second or anterior antennal somite, showing external or anterior surface : a a a, 



ophthalmic cavity and foramen ; bbb, anterior antenna, cavity, and foramen. _ 



9. Same, showing internal or posterior surface : a, ophthalmic foramen ; b b, anterior 



antenna and foramen. 

 10. Posterior antennal somite, dorsal aspect ; carapace removed to show the internal 

 surface of the ventral portion of the somite : c c, posterior antennre ; ol ol, olfac- 

 tory foramen. 



Plate II. 

 Fig. 11. Diagram showing the connexion of the branchioe with the legs and the external 

 character of the branchial chamber in relation to the internal viscera: BB, 

 branchial chambei-s ; A'p Ap, apodema. 



12. Dorsal surface of carapace, showing the natviral portions into which it is divided. 



13. The carapace removed to slmw the internal structure and the relation of the 



viscera to the external marking in fig. 12. 



