214 CEYLON PEARL OYSTER REPORT. 



The fins of B. belchevi are fairly constant (fig. 3) and agree with Kirkaldy's 

 figure, except that she figures the rostral fin proportionately rather small. In ours 

 it is well developed, rather long, and more pointed than her figure shows. In 

 some specimens it is so well devekmed as to suggest a transition from B. lameolatus 

 to B. indicus. The ventral fin has fin chambers and a double series of ventral fin i"ays. 



The extent of development of fin rays appears to vary with the age of the animal. 



In the smallest specimens no ventral or dorsal fin rays are present. In those a 

 little larger, dorsal fin rays appear, the ventral ones being still undeveloped ; while 

 in those a little larger again, both series are seen. 



The fin chambers are not prolonged postanally. The nervous system and the dorsal 

 fin end conterminously, while the notochord is prolonged a little way in front of both. 



A faecal pellet adhering to the anus of No. LIII., Table II., on examination was 

 found to be rich in skeletons of diatoms, which gives an indication of the food. 



The intertentacular membrane between the oral cirri of B. belch en is low, and the 

 sensory-papillae are very large and prominent. 



The " Raderorgan " is exceptionally well developed, there being a large median 

 dorsal lobe and six lateral lobes on each side. The liver is exceptionally large and 

 extends for three-quarters of the length of the pharynx on the right side. 



Branchiostoma pelagicum, Gunther (1889). Plate I., fig. 16. 



The six specimens taken in the tow-net were found to be poorly preserved, and 

 being also extremely small, their identification is a matter of some difficulty. I think, 

 however, that there can be little doubt that they are B. pelagicum, Gunther (2). 



Of the six, one was taken in a coarse tow-net in the Indian Ocean, to the south- 

 east of Sokotra, about halfway between Perim and the Maldives, on January 16th, 

 1902 ; while the other five were taken with a fine net on January 20th, 1902, in the 

 Indian Ocean, after passing Minikoy Atoll, and therefore hetween the Maldives and 

 the Gulf of Manaar. There is therefore no doubt as to their all being truly pelagic 

 f< >rms. 



The largest measured 8 '5 millims., the smallest 4 "5 millims., the average for the 

 six being 6 millims. The greatest breadth of the largest specimen was 1 millim. 

 All six are undoubtedly the same species, and agree in detail as far as can be made 

 out. 



The notochord extends from the extreme tip of the snout to the end of the tail, 

 projecting some little way beyond the myotomes. 



The nervous system stops short of the end of the notochord at both extremities. 

 The eye spot is very large and conspicuous, far larger in proportion to the size of 

 the animal than in the other species. This, in itself, is suggestive of a pelagic life. 

 All along the nerve cord, at regular intervals, occur groups of five or six large 

 pigment spots (fig. 16). This is a character noted in Gunther' s B. pelagicum. 



The number of myotomes (as nearly as could be determined from the imperfect 



