564 THE ANIMAL KINGDOM. 



where there are three. They are either hinged at their base or the bases 

 of the valves cross one another like the blades of a pair of scissors, and 

 they are opened and closed by special muscles. The length of the valve 

 is greater than its breadth, except in the form known as valvulate pedi- 

 cellariae. 



A series of paired vertebral or ambulacra! ossicles, really sub-ambulacral 

 by position, underlies the ambulacral grooves in their whole extent. The 

 ossicles are narrow rods with flattened sides which are closely fitted to one 

 another, and correspond one to the other on opposite sides of the grooves 

 except in the Palaeozoic Encrinasteriae, in which they alternate. A single 

 series of pores is formed by the juxtaposition of grooves on the flat sides 

 of the rods. These pores are arranged in a straight line in the majority. 

 Sometimes after the first three they are disposed in a zigzag line, i. e. 

 are alternately near and remote from the outer ends of the rods. These 

 ends lie just below the ectoderm at the edges of the ambulacral groove. 

 The inner ends of each member of a pair which are moveably articulated 

 together, meet dorsally to the radial water-vascular vessel, perihaemal 

 spaces, bloodvessel, and nerve. A muscle runs ventrally from each right 

 to each left ossicle, and the contraction of these transverse muscles 

 deepens the groove. A single series of adambulacral ossicles lies at the 

 outer ends of the (sub)-ambulacrals, each ossicle corresponding to the 

 interval between two of the latter. And externally to the adambulacrals 

 is a variable number of series of intermediate ambulacral ossicles, inter- 

 vening between the adambulacrals and the ventral marginal ossicles when 

 present. The members of the first pair of ambulacral and adambulacral 

 ossicles form the mouth plates or oral angle pieces. These parts are modified 

 in one of two ways. Either the ambulacral ossicles are the more prominent 

 while the adambulacral remain small, e. g. in Asterias ; or vice versa, as in 

 the majority of Asteroidea. In the first instance the oral angle piece 

 projects across the mouth radially, in the second interradially ; and the 

 two types may be .distinguished as ambulacral and adambulacral respec- 

 tively. The arms are usually five in number, rarely more, as in Solaster, 

 Ar chaster, Brisinga. They can be bent ventrally as well as sideways by 

 special muscles, and their tips where the eyes are situated are normally 

 curved slightly upwards. They are extensions of the disc, broad at the 

 base ; and the relative proportions of disc to arms is variable. The inter- 

 radial portion of the disc is consequently much extended or much reduced, 

 and in the latter case there are sharp interradial angles to which correspond 

 internal folds. Brisinga alone has a small disc and long arms sharply 

 marked off from it, and therefore wears an Ophiuroid aspect. 



The dorsal perisoma, except in Brisinga, developes numerous minute 

 and delicate processes. These are tubular and contractile. They contain 

 an extension of the coelome, and are known as dermal branchiae. The 



