508 A. MILNES MARSHALL. 



The disc consists of a calcareous cup or calyx (vide fig. 1), 

 and of the visceral mass which is lodged within the cavity 

 of the calyx, and contains the whole of the alimentary canal 

 and important parts of the vascular, sensory, and other 

 systems. 



The surface of the visceral mass covered by the calyx is 

 commonly called the dorsal or aboral, the opposite one 

 being the ventral or oral surface. In, or near, the middle of 

 the latter is the mouth (fig. 1, r) j this leads into a convoluted 

 alimentary canal (s) which ends in an anus placed at the top of 

 a conical chimney-like projection, which arises from the oral 

 surface of the disc not far from its edge and interradially, i. e. 

 between two pairs of arms. 



To the dorsal surface of the calyx are attached from twenty 

 to thirty jointed filaments or cirri (fig. 1, p) by which the 

 animal attaches itself to foreign bodies. The calyx itself 

 consists of a number of calcareous plates arranged as follows 

 (cf. figs. 1 and 3) : — In the centre is a single pentagonal centro- 

 dorsal plate (C. D.), to the dorsal surface of which the cirri are 

 attached, while the ventral surface is hollowed out in its centre 

 to form a cup-shaped cavity closed above by a thin calcareous 

 plate — the Rosette (R.) ; more peripherally the centrodorsal 

 plate supports a ring of five plates called First Radials (R-i). 

 To the outer surfaces of these are connected five Second 

 Radials (R2) which overlap and almost entirely conceal the 

 First Radials from the dorsal surface (fig. 1), and beyond the 

 second comes a set of five Third Radials (R.3). 



Each Third Radial bears distally apair of First Brachials 

 (figs. 1 and 3, Br.), which are the first of a series of short 

 calcareous joints placed end to end and extending the whole 

 length of the arms. 



The spaces between the radials and between the basal joints 

 of the arms as far as the fourth brachials are filled up by 

 uncalcified portions of the perisome or body wall which thus 

 complete the calyx. 



and for a very excellent summary of recent researches, vide P. H. Carpenter, 

 " The Minute Anatomy of the Brachiate Echiuoderms," this Journal, vol. xxi. 



