396 



COMPARATIVE ANATOMY 



CHAP. 



The straight pedicellarire are far less numerous, and are found scattered over tlie 

 integument either singly or in groups. 



The three-bladed pedieellariiB are ahvays found entirely isolated, and may be 

 altogether wanting in some individuals. 



Structure of the straight pedicellariae (Fig. 342, H). — Each of the two blades 

 consists of a hollow toothed .skeletal piece, which articulates with a common basal 



Fig. 343.- A portion of an arm of Asterias stichantlia, Sladen, from the lower side (after , 

 Sladen). 1, 2, 3, 4, The four longitudinal rows of amh.ulacral feet ; 5, fovficiform pedicellaria; ; 6, 

 adanibulacral spines ; 7, papula; ; 8, inframarginal spines ; 0, forcipiforni pedicellaria; at the 

 outer bases of these latter. 



piece. Two muscles serve for opening the pedicellaria, the outer side of each blade 

 being attached by a muscle to the basal piece. The blades are closed by means 

 of two nuiscles which run from the inner sides of their bases to the basal piece, 



Fici. 344.— Asterias (Stolasterias) volsellata.— Adambulacral plates and neishbouring portion 

 of the oral iiitegument of an arm. / , Straight ; /c, crossed pedicellaria^ on a cushion at the base of 

 a large spine (nc); «ci, smaller spine (after Sladen). 



and also jierhaps by means of two muscles which, lying for the greater part 

 within the calcareous blades, run from their tips to the basal piece. Each pedi- 

 cellaria is surrounded by a layer of connective tissue, and covered by body epithelium, 

 in which glandular cells are scattered. 



Structure of the crossed pedicellariae (Fig. 342, K). — A crossed pedicellaria 

 is not unlike a forceps with short handles. It also consists of three pieces, the 



