212 ZOOLOGY 



oids, echinoids, crinoids, etc.), though in the sea-cucumbers 

 (holothuroids) it is horizontal and much elongated. Starfish 

 are flattened vertically, as are the sand-dollars, but many of the 

 urchins (echinoids) are dome-shaped. The antimeres are at 

 right angles to this chief axis. In addition to this dominant 

 radial symmetry, there is seen even in the adult a suggestion 

 of the bilateral condition. The madreporic body generally 

 occurs in only one interradius, and a plane passing through 

 it and splitting the opposite arm divides the body into two sym- 

 metrical halves. No other plane does this. The two arms 

 embracing the madreporic body are known as the bivium, the 

 remaining three, the trivium. In some of the echinoids the 

 bilateral symmetry becomes much more pronounced than in 

 starfish. 



244. The integument consists of an outer ectodermal por- 

 tion which is often ciliated (cilia wanting in the holothuroids 

 and ophiuroids), and a subepithelial, mesodermic layer in which 

 is developed the calcareous ossicles. These may occur as spic- 

 ules, as rods, or as plates in the various classes. They may be 

 bound together by connective and muscle fibres. Frequently 

 the ossicles bear spines which may ' or may not be movable. 

 The spines are useful in defense and locomotion. Special forms 

 of spines known as pedicellaria often occur (asteroids and 

 echinoids). They consist of two- or three-pronged pincers 

 moved by muscles. They may be mounted on short stalks. It 

 is suggested that they help clear the body of foreign objects 

 which lodge among the spines. 



245. Digestive System. The mouth and anus usually open 

 at opposite poles of the principal axis (asteroids, holothuroids, 

 and some echinoids). When the axis is vertical the mouth is 

 usually directed downward, in the centre of the oral surface, 

 and the anus occupies a more or less central position on the 

 upper or aboral surface. In some of the echinoids and crinoids 

 the mouth or anus, or both, have vacated their central position 

 and may come to occupy opposite margins of the body. The 

 digestive tract is a simple tube, in the holothuroids running 

 spirally through the body. In the echinoids a similar condition 



