THE ECHINOIDEA 



315 



SUB-CLASS 3. IRREGULARIA, Desor. 



Echinoidea in whicli the anus lies outside the apical system of plates 

 in the posterior interracliu.*. 



ORDER 1. Gnathostomata, Zittel. 



Echinoidea Irregularia with a central peristome surrounded by a 

 perignathic girdle : jaws present, but sometimes rudimentary. Ambu- 

 lacra all similar. 



This is the first of the two orders of Irregular Echinoidea, and 

 differs from the other order the Spatangoida by the presence of a 

 perignathic girdle and jaws. As in the Regular Echinoids, there is a 

 marked tendency for the anus to pass backward out of the apical 

 plates. In Piigaster the peristome is much like that of Stpmechimus, 

 and the ambulacral plates are sometimes compound ; the jaws are fragile, 

 but otherwise normal. The only 

 character that excludes the genus 

 from the Diademoida is that 

 the anus opens outside the apical 

 plates (Fig. XXXV. 1). Pygaster is 

 thus the nearest form we know 

 to the ancestor of all the Clype- 

 astroid and Spatangoid sea-urchins. 



The order Gnathostomata in- 

 cludes three main series. The first 

 was typically Mesozoic, and was 

 characterised by the reduction in 

 the functional importance of the 

 jaws, and the formation of the 

 perignathic girdle into a high tubu- 

 lar peristome. From this series 

 branches diverged in opposite 

 directions. In one the jaws dis- 

 appeared and the perignathic girdle : 



became rudimentary ; while the 

 ambulacral plates remained as in Pygaster. This branch culminated in 

 the aberrant group of the Galeritidae. Later on a second branch was 

 given oft' ; in this the jaws became of greater power ; the ambulacra 

 became complex, parts of them expanding into petals, the podia of which 

 act as branchiae. The Gnathostomata may accordingly be divided into 

 two sub-orders. 



SUB-ORDER 1. HOLECTYPINA. 



Gnathostomata in which the jaws are reduced in size and strength. 

 In the most primitive members the jaws are arranged as in the 

 Diademoida ; but in later forms they are inside a tubular perignathic 

 girdle. The jaws do not work on sockets. 



Fio. XXXV. 



Irregular Echiiioklea. 1, Pygaster semisul- 



i-ntilK, Phil. ; Jurassic. :.', F.rhi unnrriif ariitntii*. 

 Leske; Chalk. 3, Galerites alliogalerus, Leske; 

 Chalk. 4, Scutella etriatula, Serr. ; 



