INTRODUCTION. 



23 



(Text-fig. 21), and the dorsal longitudinal muscles (1*1. I, fig. 7) have not yet 

 developed beyond the stage found in recent Asteroidea. 



Text-fig. 20. — Diagrammatic cross-section of the arm of Uranaster elizx. Am. Ch., ambiilacral channel ; 

 (J. sp., groove spines ; M^, longitudinal muscles between adambulacralia. 



Text-fig. 21. — Diagrammatic cross-section of arm of Stenaster obtusus. Jlf-i, dorsal longitudinal mviscles ; M^, 

 longitudinal muscles between adambulacralia ; Ifj, ventral longitiidinal muscles ; Am. Ch., ambulacral 

 channel ; cw., coelom ; Ar. p., articulating peg. 



Text-fig. 22. — Diagrammatic cross-section of arm of Aspidosoma graya. c. t.f., branch canal to tube-foot ; 

 cup t.f., cup for tube-foot ; remaining lettering as in Text-fig. 21. 



Text-fig. 23. — Diagrammatic cross-section of the arm of an Auluroid (after Sehondorf). Am., ambulacral ; Ad., 

 adambulacral ; A'^., process of ambulacral ; Wr., radial branch of water-vascular system; F., foot. 



(ii) " Ophiuroid" Structures. — That the form has progressed somewhat towards 

 the Ophiuroid (wriggling) mode of life is shown — 



(a) By the large size of the adambulacralia, which occupy as much of the depth 

 of the arm as do the side plates of Recent Ophiuroidea (Text-fig. 21). 



(b) By the presence of articulating pegs on the ambulacralia (PL I, fig. 6). 



