192 



METAZOAN PHYLA 



222. External Appearance. — A typical starfish is an animal consisting 

 of a disc from which arise five rays. The bases of these rays occupy the 

 whole circumference of the disc, but they taper to blunt points at their 

 tips. The upper, aboral surface (Fig. 101) is covered with spines, around 

 the base of which are grouped very minute organs known as pedicellariae. 

 When examined under the microscope a pedicellaria (Fig. 103 A) is 

 seen to possess two jaws which differ somewhat in different types. These 

 structures serve to rid the surface of the body of foreign objects. The 

 disc and rays may be considered as divided by imaginary lines into sec- 



TubP 

 foot 



Fig. 102. — Oral surface of a starfish, Asterias vulgaris Verrill. From a preserved 

 specimen in which the water-vascular system had been injected before preservation so that 

 the tube feet are extended. One ray turned as if the animal were beginning an effort to 

 turn over. X 14 • 



tions, the lines running from the center of the disc to the tips of the rays 

 being radii, and those running from the same point to the apices of the 

 angles between the rays being interradii. On one of these interradii 

 and sharply distinguished by its smooth appearance is a little circular area 

 which is called the madreporite. In some cases it is possible to distinguish 

 a very minute anal opening close to the center of the disc. 



On the lower or oral surface (Fig. 102), and in the center of the disc, 

 is the mouth, which is surrounded by a soft membrane, the perioral 

 membrane, or peristome, bounded on the outside by a ring of calcareous 

 plates or ossicles. From this ring an amhulacral groove (Fig. 103 A) 

 runs out the middle of each ray to the tip, the width of this groove being 

 less than the width of the ray but leaving only a relatively narrow strip 

 along each side. The margin of this groove is furnished with longer 



