230 ANNULOIDA. 



for the attachment of the muscular fibres which move the 

 spine. 



The " apical disc " or " genital disc " occupies the summit 

 of the test, and is generally composed of ten plates (figs. 117, 

 c, and 118, B). Five of these plates are of comparatively 

 large size, and are termed the "genital plates," each being- 

 perforated by the duct of an ovary or testis. Each genital 

 plate occupies the summit of one of the interambulacral 

 areas. One of the genital plates (the right antero-lateral 

 plate) is larger than the others, and supports a spongy 

 tubercle, perforated with many apertures, like the rose of a 

 watering-pot, and termed the " madreporiform tubercle " (fig. 

 118, B). This structure protects the mouth of the canal by 

 which water is admitted from the exterior to the water- 

 vascular system. Wedged in between the genital plates, and 

 occupying the summits of the ambulacra! areas, are five 

 smaller, heart-shaped, or pentagonal plates, each of which 

 is perforated for the reception of an " ocellus " or eye, and 

 which are therefore termed the " ocular plates." 



The EcJiinoidea may be divided into the four groups of 

 the Eegular Echinoids, the EchinotJmridce, the PeriscJioecki- 

 nidce, and the Irregular Echinoids ; and we may briefly 

 consider the more important characters, the geological range 

 and the leading type of these divisions. 



I. EEGULAR ECHINOIDS (EcJiinoidea endocyclica). In this 

 group of the Echinoids the mouth is situated in the centre 

 of the base (fig. 123), and the vent is placed at the summit 



^^^^^^^^ 



Fig. 123. Salenia penottata, t "regular" Echinoid. The left-hand figure represent* 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. 



of the test, surrounded by the genital disc. They have a test 

 which is almost always circular, or spheroidal, or, it may 

 be, depressed ; and the mouth is armed with a complicated 



