ASTERIAS FORBESI 175 



madreporic plate. The two arms between which the madreporic 

 plate is situated form the bivium, the remaining three the trivium. 

 The entire dorsal surface is covered with short spines or tubercles 

 surrounded by groups of pedicellariae. On the ventral surface, 

 in the middle of the disc, is the mouth surrounded by five pairs 

 of oral spines. Radiating from the mouth are the ambulacral 

 grooves in which the ambulacral feet lie in four rows in each arm. 

 The edges of the grooves are covered with sharp spines. At the 

 end of each arm, imbedded among spines, is the terminal tentacle 

 with a red spot at its base the eye. 



The body wall is very thick and hard owing to the presence 

 of calcified plates or ossicles 1 in its inner layer. The ossicles of 

 the dorsal surface are more or less irregular in shape and are 

 held together by connective tissue and muscular fibres. The 

 ventral surface is formed by articulated ossicles arranged in four 

 rows in each arm. The two middle rows are formed by the 

 ambulacral ossicles articulated with each other in the median 

 line. Between these are the ambulacral pores through which the 

 ambulacral feet project. The two outer rows of ossicles are called 

 adambulacral and they carry the sharp spines already men- 

 tioned. The peristome is surrounded by a 'pentagon of oral 

 ossicles. The outer layer of the body wall is formed by a ciliated 

 epithelium. Outside of the adambulacral plates on the ventral 

 surface and on the dorsal surface are numerous branching (or 

 papulae) protruding through the interstices between the ossicles. 

 They are short, tubelike projections of the ccelome or body cav- 

 ity, are lined on the inside with ccelomic epithelium and are 

 capable of being completely retracted. They serve for the 

 purpose of respiration. The pedicellarice are little scissor-like 

 appendages sitting around the tubercles and spines. They are 

 composed of three parts, two blades or jaws and a basal plate 

 with which the blades are articulated. There are two kinds of 

 pedicellariae, larger ones with straight edges of the blades and 



1 The entire skeleton of ossicles in the starfish is so articulated that it 

 admits an extensive downward and very slight lateral motion. 



