OF THE ANNULOSA. 387 



among the Crustacea, and which are replaced in the true 

 Arachnida by organs of a totally different construction. 



But, granting this doctrine of M. Latreille to be agree- 

 able to nature, it may next be urged that the greatest argu- 

 ment for the necessity of distinguishing the Parasita or 

 Anoplura of Dr. Leach from the Arachnides tracheennes 

 of Latreille is thus done away with, since both possess a 

 pair of antennae. Nay, M. Latreille seems to have him- 

 self judged that it ought to have this consequence; for in 

 his last general distribution of the Annidosa, published in 

 the Annales du Museum, he places the parasitical Ame- 

 tabola in the class oi Arachnida under the name oi Arach- 

 nides pediculaires. Some persons, however, may be dis- 

 posed to think that in doing this he too hastily abandons 

 his old arrangement, which the new theory, once admitted, 

 instead of weakening will serve to establish beyond a doubt. 

 Thus it has not escaped him that the two antennae of his 

 Arachnides ptdiculaires represent the lateral pair of an- 

 tennae in Crustacea : now, these are the two which remain 

 in the Onisci, but absolutely become null in the true 

 Arachnida. So that, in following the changes which the 

 antennae undergo in the Annulose animals, a most beau- 

 tiful I'egularity presents itself to the view. A Decapod 

 Crustaceous animal has, for instance, four antennae, the 

 middle pair of which disappears in Oniscus ; from which 

 circumstance, in the circles of Ametabola and Hexapod 

 insects we have only one pair of antennae, which answers' 

 to the lateral pair of Crustacea, and finally disappears in 

 Ni/cteribia. If again, on the other side, we quit the 

 Crustacea, by means of the Pycnogonida, it is the external 

 or lateral pair which is most ready to disappear ; and 

 among the Arachnida we discover only the intermediate 



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