ARTHROPODA 287 



separated and connected by about ten transverse strands in each 

 segment. There are sHght enlargements in each segment which can 

 be regarded as incipient gangha, but the whole nervous system is 

 primitive for an arthropod or even an annelid and can be best 

 compared to that of Polygordius in the annelids or Chiton in the 

 molluscs. 



The Onychophora are not known as fossils, but all that has been 

 said indicates that they came off from the main arthropodan stock at 

 a very early stage when a typical haemocoele had been developed and 

 cephalization had commenced, but when the epithelium had not finally 

 specialized in the production of chitin and was still ciliated in places. 



SuBPHYLUM TRILOBITA 



Palaeozoic Arthropoda with the body moulded longitudinally into 

 three lobes; one pair of antennae; and, on all the postantennal so- 

 mites, appendages of a common type which has two rami and a 

 gnathobase. 



The Trilobita were marine organisms and were very numerous in 

 the Cambrian and Silurian but became extinct by the Secondary 

 period. Their body was oval and depressed, and" 

 consisted of a head and a segmented trunk, of 

 which the anterior somites were movable on 

 one another, but the hindermost, in varying 

 number, were nearly always united to form a 

 tagma known as the pygidium. The body could 

 usually be rolled up like that of a woodlouse. 

 Along its whole length longitudinal grooves Fig. 208. Olenmcata- 

 divided lateral pleural portions from a middle ractes, from the Lin- 

 region. In the head, this middle region is known ^.^^ Flags. Natural 

 as the glabella and transverse furrows usually ^^^^* ^°"^ 

 mark out more or less distinctly five somites. The pleural portions 

 of the head are known as the cheeks^ and each bears in most species 

 a sessile compound eye. On each cheek a longitudinal facial suture 

 divides an outer from an inner area, passing immediately internal to 

 the eye. The posterolateral angles of the cheeks are often produced 

 backward as spines. Under the head a large labrum or hypostoma 

 projects backward below the mouth, behind which is a small 

 metastoma. 



The antenna is uniramous and multiarticulate and is the only pre- 

 oral appendage. Since it is the foremost of five head appendages it 

 has the same position as the antennule of the Crustacea, with which 

 it is probably homologous. In that case it would seem likely that a 

 true first somite had already, as in modern Crustacea, become merged 



