TO THE EXISTING ARTICULATA. 39 



stitutes this group, corresponds only in some peculiarities of the cephalic shield or of the 

 cephalothorax with the Trilobites, and for this reason, and since it is only a partially analogous 

 form, I shall not say more concerning the group. 



Of the two other groups, the Lopltyropoda generally exhibit a slighter or less compact 

 form of the body, and rather large antenme ; they also have cither a single eye, (which in 

 some species is large, in others small), or two very small eyes, and therefore exhibit 

 characters which correspond less completely with the type of the Trilobites than the more 

 considerable size of body, larger eyes, and undeveloped antennae of the PhyUopoda; I 

 therefore do not hesitate to recognize in the latter the nearest affinities of the Trilobites, for 

 which reason I shall now describe their organization more particularly. 



SECTION XIX. 



The Phi/Uopoda have a soft, fleshy body, the thorax generally consisting of eleven 

 members, bearing the same number of fin-shaped organs of motion. The head is an 

 independent division, to which, besides the antennae and organs of mastication, a rudi- 

 mentary pair of feet is attached, by which the number of feet of the tliorax is increased to 

 twelve (4 x 3). The other organs vary ; there are generally two pair of antennae before 

 the mouth, but these are either remarkably small, as in Ajms ;* or only one pair consists of 

 distinct antennae, whilst the other is prehensile, and assists in the act of copulation, as in 

 BrancMpm ; or else, lastly, the former pair of antennae assists in locomotion as a fin-foot, 

 while the latter is a short, almost jointless, lobe of flesh, as in Limnadia. The eyes present 

 similar differences. There are two large compound eyes, and one simple eye, in all Phyllo- 

 poda, but the former are either planted on a long peduncle, and are moveable, as in 

 Brancliipus, or are immoveable, and in that case partly united in a circular form, as in 

 Limnadia, and partly arranged in two distinct semicircles, as in Apms. The simple eye 

 stands between them at the extremity of the forehead, or if they are close to each other is 

 behind them. It is remarkable, however, that BrancMpm, the genus having laro-e, greatly 

 projecting, moveable eyes, does not possess any protecting covering, whilst Ajom and 

 Limnadia are so provided, the protecting envelope in the former of these genera {Apus) 

 consisting of a head plate enlarged into a shield, which only, indeed, covers the head, but is 

 intimately connected with the body of the animal ; while in the latter, on the other hand, 

 {Limnadia), it consists of a shell formed as in shells with double flaps, and this shell is placed 

 at the junction of the head and body, (and therefore, properly speaking, also at the head,) 

 and can be opened and shut below at the will of the animal. Another important relation 

 harmonizes with the presence of this shell, namely, the structure of the abdomen, which 

 in the genera covered by shells, exhibits no marked distinction from the thorax ; and even 

 (which is the only case in the whole class of Crustacea) bears feet constructed in exactly the 

 same manner, only successively smaller, and not merely is there a single pair at each ring, but 

 at first two at each, and afterwards even three or four. The number of organs of motion is 



* Apus, Branchipus, and Limnadia, are represented in Table VI, Figs. 1, 3, and 15. 



