PALAEOSTRACA. 



333 



Xauplius stage, further, is typical of their development. The Palaeo- 

 straca, on the contrary, seem to lack both these characteristics. We 

 shall therefore, perhaps, be more accurate in our determination of 

 the systematic position of the Palaeostraca if we consider them, 

 not as true Crustacea, but as a distinct group, nearly related 

 to the Crustacea, which branched off independently from the 

 ancestral form (Protostraca) before the first typical characters (the 

 two pairs of antennae and the Nauplius stage) had developed 

 (p. 315). A feature which is very common among the Palaeostraca, 

 but does not occur to the same extent among true Crustacea, is the 

 frequent fusing of the posterior segments of the body to form a 

 distinct region (pygidium). This is evidently an adaptation to the 

 habit of rolling up that part of the body. Among the Palaeostraca, 

 the lowest grade of development is found in the Trilobites, as is 

 evident from the more 

 homonomous segmentation 

 of the post-cephalic region, 

 and, according to Walcott, 

 the uniform character of 

 the numerous limbs. The 

 same author has shown 

 (No. 5) that the limbs of 

 the Trilobites exhibit in 

 their structure remarkable 

 agreement with the typical 

 Crustacean limb. The 

 former are biramose (Fig. 

 149), with a five- or many- 

 jointed endopodite (en) ending in a claw, and a two- to three-jointed 

 exopodite (ex). On the outer side of the coxal (basal) joint, spiral 

 epipodial appendages (ep), thought to be gills, are attached.* It 

 may be pointed out that, in Limulus also, the biramose character 

 of the Crustacean limb finds expression in the presence of an 



* [The recent researches, more especially those of BexCHXB (No. II.), on the 

 limbs of Trilobites show that these appendages, with the exception of the highly 

 specialised antennae, were but slightly modified in the different body-regions. 

 Typically each appendage was biramons, consisting of a two-jointed protopodite, 

 whose coxal joint in the more anterior limbs formed a masticatory blade, i five 

 jointed endopodite, and a strongly setiparous exopodite with an expanded 1ms.i1 

 joint, and a multiarticulate palp. On the pygidium, these appendages were more 

 lamellate, and closely resemble those of the larval Apns, being typically phyllo- 

 podiform, and it is these limbs that Beechkii regards as indicative of the 

 primitive type of limb-structure. Anteriorly, the endopodites assume a more 

 cylindrical form. — Ed.] 



Fig. 140. — Diagrammatic cross-section through a 

 trunk -limb of a Trilobite (after Walcott, from 

 Lang's Text-book), en, endopodite; ep, epipodial 

 appendages; ex, exopodite; d, intestinal canal; 

 r, tergum ; p, pleura. 



