ANAL SYSTEM. 685 



In some of the Spatangoids the abactinal system becomes very elongated, 

 by the great increase in breadth of the lateral posterior interambulacra near 

 the apical system. This is carried so far that in some of the fossil families — 

 Dysasteridae and Ananchytidae — the genital and ocular plates of the 

 trivium and the bivium become completely disconnected by the junction of 

 the apical plates of the lateral posterior ambulacra of opposite sides of the 

 test, thus forming two independent apical systems. We find a gradual 

 passage between the compact abactinal system of such recent genera as 

 Schizaster, the Brissina, and the like, to the disconnected apical system of the 

 Dysasteridae, through such fossil genera as Cardiaster, Holaster, Ananchytes, 

 where the plates of the apical system gradually become arranged in two 

 parallel lines, the ocular and genital plates alternating ; the system ending 

 at the anterior extremity in an odd ocular plate, and at the posterior one in 

 two posterior ocular plates. 



