THE ECHINOIDEA 



295 



In Echinus the madreporite (Fig. I. w), or the opening of the 

 water- vascular system, is on the right anterior genital plate. After 

 the anus has receded from the apical system, the madreporite 

 begins to follow it. In a simple compact apical system the pores 

 of the water-vascular aperture occur only in the right anterior 

 corner of the system. Such a system is said to be " ethmophract," 

 as in Discoidea or Micraster (Fig. XI. 4). In more advanced forms 

 the pores and the plate on which they open extend backward until 



1 I 4 NfiBaX 1 ^ '^- 



Apical systems of Echinoidea. 1, Pygaster umbrella ; 2, Clypeus sinmtus ; 3, Clypeus Hugi ; 

 4, Discoidea coniea ; 5, Spatangus purpureus. In 1 and 2 the anus is still in contact with the 

 apical system ; in 3 it is free ; 4 is ethmophract, and 5 ethmolysian. 



they separate the two postero-lateral genitals, as in Spatangus 

 purpureus (Fig. XL 5). Such an apical system is said to be 

 " ethmolysian." 



Another change in the apical system is wrought by the elonga- 

 tion of the test in the antero- posterior direction, whereby the 

 apical system becomes elongated. The plates of the two pairs of 

 genitals become adjacent, and completely separate the anterior 

 and the two pairs of oculars from each other ; the three anterior 

 ambulacra usually meet close together, and are separated from the 

 two posterior ambulacra by a wide space. The three anterior 



