434 



ECHINODERMATA ASTEROIDEA 



brought about by muscles extending from the " handle " of one 



blade to the upper part of the other. 



Scattered about amongst the papulae between the cushions 



are other pedicellariae of a larger size in which the blades do 



not cross one another (Fig. 186, B). 



In the space or " interradius " between two arms, on the aboral 



surface, there is found a button-shaped ossicle. This is covered 



with fine grooves, and 

 from a fancied resemb- 

 lance between it and 

 some forms of coral it 

 has received the name 

 " madreporite " (Fig. 

 18 5, mac?). The bottoms 

 of the grooves are per- 

 forated by capillary 

 canals lined by flagella, 

 through the action of 

 which water is con- 

 stantly being intro- 

 duced into the water- 



■Pro. 186. — View of peJicellariae of A. glacialis. A, 

 Crossed form, x 100. 1, Ectoderm covering the 

 ■whole orgau ; 2, basal ^liece ; 3, auxiliary muscle Vascular SyStem, 

 closing the blades ; 4, muscle lifting right blade out 

 of the groove ; .5, handle of left blade ; 6, muscles 

 closing the blades, and uniting to form 7, the 

 muscular string attaching the pedicellaria to the 

 skeleton. B, straight form, x 10. 1, Basal piece ; 

 2, blades ; 3 and 4, muscles closing the blades ; 

 6, muscle opening the blades. (From Cuenot.) 



The anus is situated 

 near the centre of the 

 upper surface of the 

 disc, but it is so minute 

 as to require careful 

 inspection in order to discover its position (Fig. 185). 



On the under side of the animal the most conspicuous features 

 are the five ambulacral grooves which radiate out from the 

 " peristome," a thin membranous area surrounding the central 

 mouth. The grooves are filled with the tube-feet, which are 

 closely crowded together and apparently arranged in four rows. 



Skeleton. — The sides of the ambulacral grooves are 

 stiffened by the rod-like "ambulacral ossicles." To the outer 

 ends of these are articulated a set of shorter rods termed the 

 " adambulacral ossicles " which carry each two or three rod-like 

 spines, the " adambulacral spines," the skin covering which bears 

 numerous pedicellariae (Fig. 187, B). When the animal is 

 irritated the edges of the groove are brought together, and tliese 



