ANNULOIDA: ECHINODERMATA. 139 



members of the order the test is composed of twenty rows of 

 these plates, arranged in ten alternating zones, which pass 

 from the one pole of the animal to the other, each zone being 

 composed of two similar rows. Five of these double rows are com- 

 posed of large plates, which are not perforated by any apertures 

 (fig. 37); the zones formed by these imperforate plates being 

 termed the " inter-ambulacral areas." The other five double 

 rows of plates alternate regularly with the former, and are 

 termed the "ambulacral areas," or " poriferous zones." Each 

 of these zones is composed of two rows of small plates, which 

 are perforated by minute apertures for the emission of the 

 " ambulacral tubes," or "tube-feet." Growth of the test is carried 

 on by additions made to the edge of each individual plate, by 

 means of an organised membrane which passes between the 

 sutures, where the plates come into contact with one another. 

 The plates of the test are studded with large tubercles, which 

 are more numerous on the inter-ambulacral areas than on the 

 ambulacral, and are wanting on all the plates which do not 

 belong to either area. These tubercles carry spines (fig. 38), used 

 defensively and in locomotion, which are articulated to their 

 apices by means of a sort of " universal," or " ball-and-socket," 

 joint. Occasionally a small ligamentous band passes between 

 the head of the tubercle and the centre of the concave arti- 

 cular surface of the spine, thus closely resembling the " round 

 ligament " of the hip-joint of man. Besides the main rows of 

 plates just described, forming the so-called "corona," other 

 calcareous pieces go to make up the test of an Echinus. The 

 mouth is surrounded by a coriaceous peristomial membrane, 

 which contains a series of small calcareous pieces, known as 

 the "oral plates;" whilst a corresponding series of "anal 

 plates " is found in the membrane surrounding the opposite 

 termination of the alimentary canal. Surrounding the aperture 

 of the anus at the summit of the test is the " apical disc," 

 composed of the so-called genital and ocular plates (fig. 37, 3). 

 The "genital plates" are five large plates of a pentagonal form, 

 each of which is perforated by the duct of an ovary or testis. 

 One of the genital plates is larger than the others, and supports 

 a spongy tubercle, perforated by many minute apertures, like 

 the rose of a watering-pot, and termed the " madreporiform 

 tubercle." The genital plates occupy the summits of the inter- 

 ambulacral areas. Wedged in between the genital plates, and 

 occupying the summits of the ambulacral areas, are five smaller, 

 heart-shaped, or pentagonal plates, known as the "ocular 

 plates," each being perforated by a pore for the reception of 

 an " ocellus " or " eye." 



