ECHINOIDEA. 



209 



When the anus is outside the apical system, it always lies in 

 what is called the posterior interradius (V. I). The outline of 

 an Echinoid shell when viewed from the apical pole is called the 

 ambitus. The plates of the skeleton are covered by the ciliated 

 epidermis and lie entirely superficial to the nervous and water- 

 vascular systems ; they are perforated by apertures for the 

 passage of the tube -feet, and bear prominences and tubercles 

 to which the variously shaped spines are movably articulated. 

 The apical system is very limited in extent and takes up the 

 whole of the abambulacral surface of the body (Fig. 149). The 



FIG. 148. Test of a young regular sea-urchin Strong ylocentrotus droebachiensis (from Claus). 

 a from the aboral side ; b from the oral side. PR rows of pores in the anterior radius. The 

 peristomial membrane contains the mouth with 5 teeth in the centre and 5 pairs of plates 

 perforated by pores for the oral tube-feet. 



madreporite is on the upper surface and is one of the apical 

 plates, generally the basal of what is called the right anterior 

 interradius (II, III). For purposes of description, there may be 

 said to be two principal kinds of Echinoids, the regular forms, 

 in which the body is more or less spherical and the anus is within 

 the apical system (Endocyclica, Regularia), and the irregular 

 forms, in which the body is oval, or heart-shaped, and more or 

 less flattened in the principal axis, and in which the anus is 

 outside the apical system in the posterior interambulacrum 

 (Ectocyclica, Irregularia). The test consists of the plates of 

 (1) the apical system, and (2) the corona or rest of the shell ; 

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