ECHINOIDEA 



639 



tain families the aboral ambulacral pores form a characteristic five-armed 

 figure which somewhat resembles the petals of flower and is called the 

 petaloid area (Fig. 990,4). Inasmuch as the anus has shifted its posi- 

 tion in these families to an interradius at the edge or on the oral side of 

 the body, the latter can easily be divided by a longitudinal plane into a 

 right and left half; in most spatangoids the mouth has also shifted its 

 position towards the forward end, thus increasing the bilateral 

 symmetry. 



Sea-urchins are covered with spines which are movable and articu- 

 late with small tubercles by ball-and-socket joints. In some forms the 



Fig. 986 Fig. 987 



Fig. 986 Gorgonocephalus agawlzi (Clark). Fig. 987 Test of Arbacia aboral 



aspect (Coe). 1, radius; 2, interradius; 3, madreporite ; 4, genital plate; 5, anus; 

 6, spine tubercle. 



spines are very long, and in some their tips are poisonous. A ciliated 

 epithelium covers all parts of the body, except the large spines of certain 

 forms. Stalked pedicellariae with usually three jaws occur between the 

 spines and on the peristome, the movements of which are controlled by 

 striated muscles. Sphaeridia, minute, spherical bodies which are modified 

 spines, occur in most cases on the oral surface; they are probably static 

 organs. 



Ambulacral appendages exist in three different forms: (1) loco- 

 motory feet with sucking discs, which are of very general occurrence; 

 (2) ten large oral feet which surround the mouth on the peristome and 

 may be organs of taste or smell; and (3) tactile or branchial appendages, 

 without sucking discs, which characterize the petaloid area. 



Internal Structure. The calcareous plates in the body wall, which 

 form the test, fall into three groups, the peristomal, the coronal, and the 

 apical. The corona forms the greater part of the test. It is composed 

 of twenty rows of polygonal plates (Fig. 987), which, in most sea- 



