METAZOA ECHINODERMA. 167 



The skeleton of the adult is composed of an irregular 

 network of beams and spines which are covered by a thin 

 layer often called the epidermis. This layer can be 

 scraped off with a knife, proving that the skeleton is 

 internal. The majority of the spines are immovably fast- 

 ened to the beams of the skeleton, but on the lower side 

 along each ambulacrum there are spines of quite a differ- 

 ent character from those above. These are more slender 

 and tapering, and are connected with the skeleton either 

 by cushions or by ball-and-socket joints, allowing of con- 

 siderable freedom of motion. Among the spines of the 

 starfish are little characteristic forked organs called pedi- 

 cellariae, which are spines modified for a special purpose 

 not yet known with certainty, though it is evident that 

 they are used for taking hold of objects. They are found 

 at the bases of the spines, on the soft membrane between 

 the spines, and also on the movable spines of the lower 

 side. In those Asteroidea that have four rows of tube 

 feet there are two kinds of pedicellariae ; in one the 

 blades are opposite and in the other they cross like scis- 

 sors. 



The plates of the skeleton are finely seen in the prepara- 

 tion. No. 305 (specimen on the left) shows the irregular 

 network of plates in the upper side or back, and the speci- 

 men on the right the two rows of ambulacral and interam- 

 bulacral plates in the lower side. The ambulacral plates 

 are movably articulated at the inner end. Between the am- 

 bulacral plates can be seen the four rows of holeb through 

 which pass the tube feet with sucking discs at their ends. 

 On each side of the groove formed by the ambulacral 

 plates is the row of rounded, imperforate interambulacral 

 plates which bear the movable spines already described. 

 Nos. 306-308 are preparations of the starfish, showing the 

 internal structure. The mouth leads into a stomach which 

 can be thrown over a mussel or other animal. By the 

 power of suction the food is taken in and the hard parts 

 thrown out of the mouth. A coecal prolongation is con- 



