TO THE EXISTING ARTICULATA. 47 



This peculiarity occurs, it is true, among the Aspidosfracn (a group of the second great 

 division of the Crustaceans), but only in a modified form, the difference in the numerical 

 proportion being always reducible to one fundamental number. This law is apparently not 

 observed in the case of the Trilobites. 



It would seem then that the relation existing between the Trilobites and the existing 

 Crustacea is one rather of analogy than affinity, so that the whole group may be considered 

 as a separate division, corresponding with the Jspidosfraca in the formal variation presented 

 from the typical character, but not to be looked on as a nearly allied or similar group to 

 this or to other tribes. 



Putting out of the question the important difference exhibited in the numerical pro- 

 portion of the thoracic rings just alluded to, this analogy to the Aspidostraca might certainly 

 have been considered as very close — all the other relations of organization, so far as they 

 can be traced, corresponding very accurately — if it were not for the structure of the 

 extremities. These, indeed, which are hard, horny, and articulated in a subdivision of the 

 present Aspidostraca, were probably entirely absent in this form in Trilobites ; but in other 

 respects all the typical characters of the two groups will be found to correspond. 



The present, however, appears to be the proper place to institute a still further investi- 

 gation into this subject, which may serve as an additional illustration. 



SECTION XXIII. 



It follows as a matter of necessity that the Trilobites, belonging as they did to the 

 great natural division of articulated animals, must have been subjected to a periodical 

 growth, during which their horny or stony cases were thrown off and exchanged for new 

 ones. This has been already alluded to by Wahlenberg, who has also suggested that some 

 supposed new species may have been founded upon these cast shells. I am not, indeed, 

 inclined to agree to the probability of this assumption ; but in order to illustrate my own views 

 on the subject, it will be necessary first to describe the process of exuviation and develop- 

 ment of the recent Pliyllojmdn. 



SECTION XXIV. 



All Pliyllopoda are subjected to a true metamorphosis, and that a progressive one. 

 They leave the egg as unarticulated pyriform animals, and at the anterior thicker extremity 

 of this pyriform body we perceive one simple eye, two pairs of unequal oar-shaped feet, 

 rudimentary antennae, and an organ of locomotion, in addition to the two pairs of feet which 

 subsequently converted itself into the real branchial apparatus.* The young animals are 

 always quite naked and destitute of shells, whether the parents possess shells or not. If the 



* Fig. 14, in Table VI, represents the young of a Branchipus immediately after its emergence 

 from the egg; Fig. 13, the young of Apus after the first moult — a indicates the small antenna;, b the 

 large ones, c the foot of the branchial apparatus, d the rudiments of the subsequent feet. 



