HOLOTHURIOIDEA OF THE INDIAN OCEAN. 79 



Internal Structure. — The calcareous ring is very small and 

 presents no points of interest. There are one or two long, tine, 

 Polian vesicles. The stone canal is evidently very small, as I 

 have been unable to determine its presence in several specimens. 

 Other observers have noted the presence of one or two stone 

 canals. The left respiratory tree is more bulky than the 

 right, but not so long. There are no Cuvierian organs. 



S'picules. — These consist of tables and buttons. The tables 

 when fully developed have a disc 60 [J. in diameter, consisting 

 of a central hole apparently divided into four when seen from 

 below, owing to the presence of the four beams of the tower. 

 There are also about eight smaller peripheral holes. The 

 edge of the disc is slightly irregular, as though suggesting the 

 presence of still another circle of holes in perfect condition, 

 although in some specimens the edge is quite smooth, as in 

 H. bovjensis. When seen in side view the edge of the disc 

 turns up slightly. The tower is generally short, and consists 

 of four uprights connected by a single cross beam. Occasionally 

 a fairly tall tower is seen. The top of the tower is square and 

 bears a few blunt spines. The type of table described above 

 is, however, rarely seen in the adult. The common form has a 

 disc with an irregular edge, in Avhich the outer circle of holes 

 has broken down, leaving either only four holes, one at each 

 corner, or no peripheral holes, or even no holes at all. In such 

 tables the toAver is frequently reduced and may be absent 

 altogether. The buttons are very irregular. Typically they 

 are smooth buttons with three or four pairs of holes. Often, 

 however, some of the holes are missing, and thus the charac- 

 teristically asymmetrical appearance is produced. Often the 

 buttons are slightly twdsted when seen in side view. 



The buttons are usualty arranged in groups or circles, but 

 in many of the specimens I have examined this arrangement 

 is not very clear, due probably to the contraction of the integu- 

 ment. The pedicels are supported by robust curved rods, which 

 are perforated at each end. There appear to be two distinct 

 types of spicules, as Fisher (11) states. In some specimens the 

 tables are large and well developed, and the buttons frequently 

 have four holes. In the majority of specimens, however, the 

 tables are reduced and the buttons are very irregular. 



