ECHINOIDEA. 



107 



An important division of the Echinoids is constituted by the 

 position of the anal aperture. In one great group of Echinoids 

 the mouth is situated in the centre of the base (fig. 62), and 

 the vent is placed at the summit of the test, surrounded by the 

 genital disc. These are the so-called " regular " Sea-urchins 







Fig. 62. Salenia personata, a " regular" Echinoid. The left-band figure represents 

 the upper surface of the shell, and shows the anus surrounded by the apical disc. The 

 right-hand figure shows the mouth in the centre of the base. 



(Echinoidea endocydica?). They have a test which is almost 

 always circular, or spheroidal, or, it may be, depressed ; and 

 the mouth is armed with a complicated masticatory apparatus. 

 In the second great group the mouth is situated on the lower 

 surface of the test, and is sometimes central, sometimes excen- 

 tric in position. The anus varies in position, but is never 

 placed on the summit of the test, opposite to the mouth. The 

 anus, therefore, is not surrounded by the genital disc. Most 

 commonly the anus is marginal, or is sub-marginal, coming to 

 be placed in this last case on the lower surface of the test near 

 the mouth (fig. 63). The Sea-urchins in which this state of 



Fig. 63. Discoidea cylindrica, an "irregular" Echinoid. The right-hand figure shows 

 the summit of the shell, with the genital disc. The left-hand figure shows the base of 

 the shell, on which are situated both the mouth and anus. 



parts obtains are termed the " irregular " Echinoids (Echinoidca 

 exocyclica). They are further distinguished by being mostly 

 oblong, pentagonal, heart-shaped, or discoidal in form, by 

 being mostly destitute of any masticatory apparatus, and by 

 having only four perforated genital plates. 



Another group must be constituted for the reception of the 

 Sea-urchins of the Palaeozoic Rocks, which differ materially 

 from the Mesozoic, Kainozoic, and Recent Echinoids. Only 



