169 



distance from the point. The backward projecting process from the trans- 

 verse rods seen in the figure is no constant feature. The thorns on the 

 postoral rods and apparently also on the posteroclorsal rods are very long, 

 and lend this rod a very peculiar pectinate appearance; but as they are 

 directed ventrally they can be seen distinctly only when they (or the rod 

 itself) are (is) broken. As seen in Fig. 93, B they may be bifid or even 

 trifid at their base, and there may further be a small side branch near 

 the point. Whether this complicate structure is a constant feature is hard 

 to ascertain, as it can only be seen when the rod is broken and lies in a 

 favourable position. Neither can the possibility be denied that these minor 

 differences really are specific characters, so that more than one species 

 may be included under the larval form designated here as 0. pusillus, 

 species c. This cannot be decided from the study of preserved material 

 alone. None of the other species of the 0. pusillus type appear to have 

 such long thorns on the postoral rods. - The body of the present species 

 is somewhat shorter and comparatively broader than that of the two pre- 

 ceding species. Also the frontal area is comparatively lower and the preoral 

 vibratile band less arched. 



A considerable number of specimens were found in a plankton sample 

 taken by my brother, Mr. H. Mortensen, off Colombo, Ceylon, 13/11. 

 1898. Further I have specimens from the Gulf of Aden, 3/V. 1900 (1 

 specimen), and the Malacca Strait, 30/XI 1/XII. 1899 (8 specimens). 



This species has several points of resemblance with Ophiopl. retrospinus, 

 species b, and probably they are nearly related. The possibility that they 

 are really only one, very variable species cannot even be denied. Only the 

 study of living material can decide the question. 



Species d. (PI. XXIX, Fig. 5). The shape of the body is like that of 

 species c, short and rather broad, with a very small frontal area; it differs 

 from the other species in the posterior extremily of its body being more 

 rounded. As seen in the figure the nervous system may be distinctly observ- 

 able, showing the typical arrangement 1 ). 



This species differs from the preceding ones in the shortness and rather 

 robust character of the body skeleton (Fig. 91); there is no process from 

 the recurrent rod. The end rods are short, straight, and with the small 

 side branch close to the point. The posterolateral rods are provided with 

 long thorns along the inner side, the outer side being smooth. The thorns 

 of the postoral and posterodorsal rods appear to be more bilaterally ar- 

 ranged than in the other species and are especially much less developed 

 than in species c. 



') Th. Mortensen. Notes on the development anil the larval forms of some Scandinavian 

 Echinodenns. p. 15& 160. 



22 



