PHARYNX OF AN UNKNOWN HOLOTHUEIAN. 259 



skeletal structures developed over an area, that between the 

 annulus and the water vascular ring, which remains devoid of 

 a definite skeleton in the genus Holothuria and most other forms; 

 but at all events, the differentiation between the pharyngeal and 

 annular systems of plates has here been extended so far that it 

 is convenient that they should receive distinctive terms. 



In the form of the annularia the present pharynx resembles 

 that of Cucumaria doliolum,i except that in this species 

 the radial annularia are notched below instead of being perfor- 

 ated, and that the interradial are also notched. It would appear 

 as if the perforation in the radial annularia in the present form 

 were a remnant of a similar notch closed below, or possibly the 

 notch is a degeneration from a perforation. The three ventral 

 median annularia are completely fused in Cucumaria 

 doliolum, but closely resemble in form the corresponding 

 plates of the present form. Similar diff'erentiations of the 

 ventral annularia in relation with the reduction in size of the 

 two median ventral tentacles occur in Ocnus pygmoeus, 

 Semper, and elsewhere. 



In that the series of interradial pharyngealia are not extended 

 so far posteriorly as the radial, the present pharynx agrees 

 with nearly all similarly protected, in some of which inter- 

 radial pharyngealia may be wanting altogether. 



In the general disposition of the pharyngealia it resembles 

 two species of Cucumaria and one of Thyone figured by Semper,^ 

 and also Thyone sacellus, figured by Selenka,^ excepting 

 that in all these instances there are always wide intervals closed 

 only by membrane intervening between the two rows of pharyn- 

 gealia composing each radial series, whilst in the present instance 

 these plates interlock with one another dii'cctly by their edges. 



Further, in Thyone sacellus the annularia appear as yet 

 indistinctly differentiated from the pharyngealia, being com- 

 posed of minute separated plates and ossicles. It is difficult to 

 compare closely the arrangement of the minute plates forming 

 > Selenka, ' Z. f. W. Z./ xvii, Taf. xx, fig. 108. 



2 Semper, 1. c, p. , Taf. xiv, figs. 4—6, Taf. xv, fig. 7. 



3 Selcuka, 1. c, Taf. xx, fig. 115 



