CHAPTEE XX. 



PALAEOSTRACA. 



Under the name Palaeostraca, Steinmann and Doderlein have 

 imited the groups of the Trilobita, the Gigantostraca, and the 

 Xiphosura. There can hardly be any doubt, as Dohhn {Xo. 11) 

 and others have shown, that these three groups are closely connected. 

 The Xiphosura (among Avhich the genus Limulus, as sole living 

 representative, is of special interest) show, in the shape of the 

 cephalo-thorax, especially in Belinurus, a striking resemblance to 

 the Trilobites, which are also recalled in the ontogenetic stages of 

 Limulus. On the other hand, the Gigantostraca {Eicrypterus, 

 Pterygotus), in the division of the body into regions and in the 

 structure of the limbs, are closely allied to Limulus. Like the 

 latter, they possess an anterior cephalo- thoracic region, with six 

 pairs of limbs, some, being chelate, taking part, by means of their 

 broadened coxal portions, in the work of mastication. The masti- 

 catory region is bounded posteriorly by a lower lip known as the 

 metastoma (in Limulus by paired rhilaria, Fig. 158, ch, p. 345, and 

 Fig. 159 B, p. 346). The region which follows the cephalo-thorax, 

 the pre-abdomen, consists of six segments, the leaf-like limbs 

 of which served for respiration, while there follows posteriorly a 

 post-abdomen, consisting of six limbless segments and the telson. 

 This latter region, in Limulus, is in a reduced condition. If we 

 attempt to classify the Palaeostraca under the Crustacea, as has 

 repeatedly been done, we shall be compelled to widen our conception 

 of the latter group. The Crustacea, as a group, appear to be charac- 

 terised by the possession of two pre-oral pairs of antennae, which, 

 in the adult, take no part in the work of mastication, and besides 

 their locomotory function serve chiefly as sensory organs.* The 



* Tlie Crustacea are fin-ther distinguished from the Palaeostraca by the 

 structure of the uiouth-parts, wliich are developed typically in the form of 

 mandibles and maxillae, and in tlie position of tlie paiied lower lip (parag- 

 iiatlia), whicii appjears in various groups l>ehiud the juandibh^s and in front 

 of the maxillae. In the adult Crustacean, the maxillae can hardly be regarded 

 as locomotory organs. 



