CHAP. X. CLASSIFICATION. 107 



and Cycloids, according as the posterior margin of their 

 scales is denticulated or smooth, a circumstance the 

 importance of which to the animal must be infinitely 

 small, in comparison to the peculiarities of the dentition, 

 formation of the fins, number of vertebrae, &c. 



And, to return to our Class of the Crustacea, has any 

 particular attention been paid in their classification 

 to the distinctions prevailing in the " most essential 

 organs " ? For instance, to the nervous system ? In the 

 Corycaeidae, Claus found all the ventral ganglia fused 

 together into a single broad mass, and in the Calanidae 

 a long ventral chain of ganglia, the former, therefore, 

 in this respect resembling the Spider Crabs and the 

 latter the Lobster; but no one would dream on this 

 account of supposing that there was a relationship be- 

 tween the Corycaeidae and the Crabs, or the Calanidae 

 and the Lobsters. Or to the organs of circulation ? 

 We have among the Copepoda, the Cyclopidae and 

 Corycaeidae without a heart, side by side with the 

 Calanidre and Pontellidae with a heart. And in the 

 same way among the Ostracoda, the Cypridince, which. 

 I find possess a heart, place themselves side by side 

 with Cypris and CijtTiere which have no such organ. 

 Or to the respiratory apparatus ? Milne-Edwards did 

 this when he separated Mysis and Leucifer from the 

 Decapoda, but he himself afterwards saw that this was 

 an error. In one Cypridina I find branchiae of con- 

 siderable size, which are entirely wanting in another 

 species, but this does not appear to me to be a reason 

 for separating these species even generically. 



