264 PALEONTOLOGY 



space, the outline of which has ten notches, which mark 

 the position of external gills. This space in fossils is 

 colloquially termed the mouth, but that is not strictly 

 correct, the real mouth being smaller, and surrounded by 

 a membrane (fieristome), the plates in which are loose and 

 on its decay fall away, leaving a large space. The mouth 

 was furnished in life with a complex series of biting 

 organs (jaws and teeth) forming the lantern of Aristotle. 

 This is rarely preserved, but its existence is shown by 

 the presence of the perignathic girdle, a series of internal 

 arches arising from the lowest plates of the corona, and 

 serving as an origin for the muscles inserted on the 

 lantern. 



Towards the aboral pole the corona is surmounted by 

 a flat series of ten plates called the apical disc (or better, 

 apical system), in the centre of which is a space, collo- 

 quially called the anus, but more correctly the periproct 

 (for a similar reason to that of the peristome). Of 

 the ten plates, the five smaller are at the ends of the 

 ambs and are called the ocular plates ; the five larger 

 interradial plates are termed genital^ because they bear 

 the apertures of the genital ducts. One of the genital 

 plates, in addition to the genital pore, is perforated by a 

 great number of smaller pores, through which water 

 filters into the water-vessels: this plate, called the 

 madreporite, constitutes the only imperfection of five- 

 rayed symmetry in the test of Hemicidaris. The ocular 

 plates have been termed radials, and the genital, basals ; 

 but they are certainly not strictly homologous with the 

 plates so named in crinoids, which do not encircle 



