264 PROFESSOR 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 ahoral, the opposite one being the ventral or oral 

 surface. In, or near, the middle of the latter is the mouth (fig. 1, r); 

 this leads into a convoluted alimentary canal (s) ending 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. l,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 (CD.), 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 (R. 2 ) 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. 8 ). 



Each Third Radial bears distally a pair 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. 



The several joints of the arms are moveable on one another. Move- 

 ment towards the oral or ventral surface, which will be .called flexion, 

 is effected by muscles (figs. 1 and 2, u) running between the successive 

 segments ; extension or movement towards the dorsal surface is on the 

 other hand almost entirely due to the action of elastic ligaments 

 placed nearer the dorsal surfaces of the segments. 



The dorsal and lateral surfaces of the arms are covered by an 

 extremely thin layer of integument, but along the ventral surface the 

 soft parts are much thicker and exhibit considerable complexity of 



