ASTEBOIDEA. 



173 



plates by muscles and are covered with a glandular and nervous epider- 

 mis. The scales are flattened spinelets. The granules are short rounded 

 spinelets ; they occur set closely together on the skeletal plates. 



The pedicellariae are pincer-like calcareous structures consisting of two, 

 rarely three, blades articulated to a plate in the dermis, and capable of 

 executing snapping movements by means of a special set of muscles at- 

 tached to their base. They are contained in the dermis and are covered 



iFio. 123. Pedicellariae of Asteroids (from Lang). A-F grouped spines resembling p.edice 

 lariae. G sessile pedicellaria of Gymnasteria earinifera (after Cuenot). H stalked straight 

 pedicellaria, diagrammatic (after Cu6not). J basal piece of a stalked crossed pedicellaria 

 of Asterias rubens (after Perrier). K stalked crossed pedicellaria of Asteri%s glacialis 

 (after Cuenot). 1 calcareous blade of forceps ; 2 basal piecs ; 3 occlusor muscle ; 3 axial 

 muscle of the blades attached to the basilar piece and acting as an occlusor muscle ; 4 

 opening muscle ; 5 axial band ; in K this band divides, each branch being inserted into the 

 base of one of the blades in such a way as to strengthen the grip when the pedicellaria is 

 pulled ; 6 ectoderm ; 7 body wall ; 8 stem. 



by ectoderm. They may attain a size of 4 mm., but are usually much 

 smaller. They are modified spines, and sometimes small groups of spines 

 are so associated that they can be moved towards one another and act like 

 pedicellariae (Fig. 123, A-F). Pedicellariae are entirely absent in the 

 genera Solaster, Echinaster, Cribrella, Mithrodia, Ophidiaster, Scytaster, 

 Astropecten. 



Pedicellariae are of two kinds, sessile and stalked. Sessile pedicellariae 

 .(Fig. 123, O) arise direct from the integument, to one of the plates of which 



