396 



COMPARATIVE ANATOMY 



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

 integument either singly or in groujjs. 



The three -bladed pedicellari* are alwaj's 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 iiicce, which articulates with a common liasal 



Fio. 343.- A portion of an arm of Asterias stichantha, Sladen, from tlie lower side (after 

 Sladen). l, 2, 8, 4, The four longitudinal rows of ambulacral feet ; 5, forficlform pedicellari* ; t;, 

 adaiubulaeral spines ; 7, papulifi ; S, infraniarginal spines ; P, forcipiforni pedicellarice at the 

 outer liases 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 nniscles which run from the inner sides of their bases to the basal piece, 



Fig. 344.— Asterias (Stolasterias) volsellata.— Adambulacral plates and neighbouring portion 

 of the oral integument of an arm. /, Straight ; fc, crossed pedicellari;e on a cushion at the base of 

 a large spine (ac) ; aci, smaller spine (after Sladen). 



and also perhaps by means of two muscles which, lying for the greater jwrt 

 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 



