164 



the ventral part of it could be discerned). There is a distinct suboral cavity, 

 as in 0. similis. It is not represented in species b, owing to the less satis- 

 factory preservation; but it is distinctly discernible also in that species. 

 The body skeleton is of the compound type; there is a more or less 

 developed median process from the transverse rods. The posterolateral rods 

 are provided with strong thorns along their inner side, the other rods have 

 fine thorns. 



Fig. 90. Skeleton of Ophiopluteus retrospinus, species b. -%. 



Species a. (PI. XXII, Fig. 1). The anterolateral, the postoral and the 

 posterodorsal arms are about 1 1 / 2 the body length. The backward processes 

 from the posterolateral rods are very prominent and conspicuously ser- 

 rated, especially along their inner side (Fig. 89). Besides these there is a 

 smaller process (more rarely two, or none at all) from the lower end of 

 the body rod, also a little serrate. The end rods are of moderate length, 

 curving outwards in the lower part; the small side branch is at some 

 distance from the point. The transverse rods are hardly swollen at the 

 point, with merely an indication of interlacing branches. The median rod 

 is long, slightly thorny, the ventral rod somewhat longer than the dorsal 

 one. The rods of the inner arms are only faintly thorny, also the antero- 

 lateral rod carries only small thorns along its outer side. 



This larva was found in the Gulf of Siam, S. of the island Koh Chuen, 

 3/III. 1900 (2 specimens), W. of Koh Kong, 25/1. 1900 (1 specimen), 



