140 



Specimens of this species were found in plankton samples from the fol- 

 lowing localities: Off Koh Kong, Gulf of Siam, 25/1. 1900 (1 specimen); 

 offKoh Chuen, Gulf of Siam, 3/1 1 1. 1900 (2 spec.); 7 2' N. 75 32' E. 22/XI. 

 1899 (1 spec.); 7 37' N. 73 34' E. 26/IV. 1900 (1 spec.); 8 9' N. 71 51' E. 

 27/IV. 1900 (1 specimen, peculiar through the outer fourth part of the 

 posterolateral rods being straight); 6 40' N. 77 30' E. 2/1. 1914 (1 spec.); 

 off the East end of Sokotra, 28/XII. 1913 (1 spec.); 12 25' N. 46 E. 26/XII. 

 1913 (1 spec.). Also the larva in metamorphosis, figured in PI. XXIV, 

 Fig. 3, from the Malacca Strait, off Pulo Pisang (1/XII. 1899) must be 

 referred to this species. Another specimen having very nearly completed 

 its metamorphosis, only with the long beautiful posterolateral arms still 

 intact, was found in a plankton sample taken by my brother, Mr. H. Mor- 

 tensen, at Aden, in February 1898. 



In a plankton sample from off Pulo Pisang, Malacca Strait, 1/XII. 1899, 

 there is a young specimen differing from the typical form through the 

 posterolateral rods being slightly concave at their base (Fig. 67, B); also 

 the body skeleton is slightly different from that of the type. Perhaps this 

 may represent another species. 



One specimen from the Malacca Strait (5 58' N. 95 43' E. 28/XI. 1899) 

 shows a characteristic feature in the postero-lateral rods, the lower part 

 being distinctly thicker than the outer part, an abrupt narrowing being 

 found a little beyond the beginning of the undulating part (Fig. 67, C). 

 Whether this is only an individual abnormality or it represents a separate 

 species, cannot be stated from the present material; the same, of course, 

 holds good for the specimen mentioned above with the unusually formed 

 lower part of the posterolateral rods. 



As seen from PI. XXIV, Fig. 3 this larva belongs to the type of Ophi- 

 uroid-larvae, where the right anterolateral arm is preserved for some time 

 after the other inner arms have disappeared during the process of meta- 

 morphosis. It also presents the very interesting feature that the vibratile 

 band of the posterolateral arms continues directly across the body, below 

 the developing Ophiuroid, so as to pass without interruption straight from 

 one arm to the other. The same feature is seen in the metamorphosing 

 Ophiopluteun formosus, species a, (PI. XXX, Fig. 2) and Ophiopluteus pu- 

 xillus, species b (PI. XXIX, Fig. 3), indicating the existence of a sort of 

 pupa-stage also in the Ophiuroid-larvse. (Comp. p. 125). 



A noteworthy fact is observed in the young Ophiurid in the said figure, 

 viz. the oral shields; they have appeared in four of the interradii but not 

 in the fifth, and they are not quite of equal size. It should be emphasized 

 that they make their first appearance on the ventral side, which proves 

 that it is not a general rule that they are originally formed on the dorsal 



