INSECTA; ECHINODERMATA 949 



to a terrestrial life. The former are known from the Siliiric 

 (Upper), the latter from the Coal Measures on. 



Altogether more than 300 fossil species of arachnids and 

 several thousand modern species are known. 



Insects are known from the Ordovicic graptolite slates of 

 Sweden and from the Siluric of France. They are especially well 

 preserved in the Carbonic and later terrestrial formations. The 

 Palaeozoic forms constitute a distinct group with 14 orders, all ex- 

 tinct. Two other orders were, however, also represented in the late 

 Palaeozoic, the cockroaches [Blattoidea) being especially well repre- 

 sented on account of the hard coriaceous character of the front 

 wings Or tegiiiina. The number of known Palaeozoic insects is close 

 to 1,000 species while the Tertiary and Quaternary have furnished 

 over 5,800 species. There are over 384,000 living species (Hand- 

 lirsch). 



Phylum IX — Echinodermata. The echinoderms or spiny- 

 skined animals are characterized generally by an apparently radial 

 form, by the possession of calcareous plates or sclerites in the in- 

 tegument, and by an elaborate internal structure, the most marked 

 portion of which is the highly developed water-vascular system. 

 The oldest known forms are the Cystoidea (Cambric to Car- 

 bonic) in which the body was enclosed in a calyx of irregular 

 plates, closely united by sutures and generally supported on a 

 stem. Arms were rudimentary and the respiratory and water- 

 vascular system were not pronounced. The Blastoidca (Ordovicic 

 to Carbonic) were more regular in the arrangement of plates and 

 were armless. The calyx was, however, provided with five petaloid 

 amhulacral areas radiating from the mouth. The Crinoidea (Ordo- 

 vicic to Recent) were mostly stemmed, though some had the power 

 of separation in the adult. The calyx is composed of regular plates 

 generally arranged in five series and terminated by branching or 

 simple arms often of great length. The mouth of many Palaeozoic 

 forms (Camerata) was under a vaulted arch or tegmen, and the 

 anus was often placed at the end of a tube or proboscis. The 

 brittle-stars and starfish have the body cleft into five or more 

 movable rays, which are supplied with branches from the water- 

 vascular system and diverticula from the other body organs. The 

 branch begins in the Ordovicic. but has few fossil representatives. 

 The sea-urchins or Echini, on the other hand, are abundantly repre- 

 sented in the Mesozoic and later strata. Palaeozoic forms occur as 

 early as the Ordovicic (Bothriocidaris). In them the body is gen- 

 erally covered by a large number of plates, which, however, fall 

 into ten zones, five amhulacral, with plates pierced for the tubed 



