INTRODUCTION. 



IB 



lacralia then meet across and disguise the interior structure. The fact is worthy 

 of mention, as the closed groove causes difficulties in the investigation of many 

 fossil forms. 



As we are looking downwards the high arch formed by the ambulacralia 

 appears as a deep groove. The bottom of the groove is excavated so as to form a 

 channel. This channel I propose to call the " ambulacral channel." Along the 

 ambulacral channel runs the radial branch of the water-vascular system. The 

 ambulacralia are in opposite pairs, and follow one another in series. Regularly 

 recurring side-branches of the radial water-vascular vessel, at the extremities of 

 which are the tube-feet, lie between every two ambulacralia. The pores at the 

 outer edge of the groove are for the passage of the ampulte into the interior of 

 the body-cavity. The groove is bordered by the adambulacral ossicles, which 



PROXIMAL. 



p.M.{2) i). If.(l) w. 



d. M.{2) a 



DISTAL. 



DISTAL. 



Text-fig. 6 (on left). — An apical view of two pairs of ambulacralia and adambulacralia of ArcJiasier typicus. 



Text-fig. 7 (on right). — Apical view of two adambulacralia of Archaster typicus with ambulacralia removed. 

 2). M.(2), process for insertion of dorsal longitudinal muscle; d.M.{2), depression for same muscle; 

 p.M.{l), ridge for insertion of dorsal transverse muscle; Ap., apophysis of ambulacral; i, facet on 

 adambulacral for apophysis of ambulacral; tv., wing of ambulacral; ii., facet on adambulacral for wing of 

 ambulacral; M.{3}, depression for insertion of adambulacral longitudinal muscle. 



are buttressed at the sides by the ventrolateralia. The stout marginalia bind the 

 skeleton into a firm whole. Text-fig. 4 shows an enlarged oral view of two 

 pairs of ambulacralia and adambulacralia of Archaster ti/picus (Miiller and 

 Troschel). The adambulacralia may be seen bordering each side of the groove. 

 They are irregularly pentagonal in shape. The outer side runs almost parallel 

 with the length of the groove. The face nearest to the groove has two facets. 

 The proximal facet is short and straight, the distal facet somewhat longer and 

 slightly concave. We shall see that the appearance thus presented is characteristic 

 of primitive Asterozoa and primitive Ophiuroidea as well as of the Asteroidea. 

 Three rows of spines, shown in the figure by spine-pits, run along the length of 

 the plate. In general the longitudinal arrangement of the adambulacral spines is 

 characteristic of the Asteroidea as opposed to the side-to-side arrangement in the 

 Ophiuroidea {vide infra, p. 16). Ludwig (40, p. 526) groups the whole of the 



