122 TGERNA B. J. SOLLAS. 



mentioned, a noticeable quantity of calcium, iron, magnesium, 

 and phosphoric acid. 



The radula after treatment with hydrofluoric acid proves 

 very resistant to prolonged boiling in caustic potash (5 per 

 cent, to 40 per cent.) ; this suggests that it is composed of 

 chitin. With a view to testing this, radul^e were treated 

 with hydrofluoric acid, boiled for a considerable time in a 

 5 per cent, solution of caustic potash, which was frequently 

 changed, and were finally extracted with water, absolute 

 alcohol, and ether. The specific gravity of the organic 

 portion of the radula which remained after these processes 

 was the same as that of chitin obtained from the carapace of 

 Astacus, and subjected to the same treatment — viz. 1'40. 

 The specific gravity was determined by a diffusion column 

 formed of chloroform and absolute alcohol. I find that chitin 

 from various sources has a specific gravity differing- but 

 slightly, if at all, from that of Astacus, and I hope to publish 

 details on this subject shortly. The refractive index of this 

 organic basis of the radula was found to lie between the same 

 limits as that of chitin, viz. 1*550 and 1"557. 



There is an interest of a general nature attaching to the 

 fact that the radula of molluscs has a chitinous basis, since 

 Bloch has brought forward special arguments (5) to prove 

 that this membrane is not produced by the direct metamor- 

 phosis of the formative cells, but by secretion. Bloch is able 

 to cite a number of workers who share his view, and he 

 refutes the arguments of Wiren, the only supporter of 

 Trinchese in the suggestion that the radula is formed by 

 direct metamorphosis of cell protoplasm. Chatin, on the other 

 hand, in 1892, maintained that the chitin of Libellnlid larvce 

 is formed by direct transformation of the chitinogenous cells. 



After soaking in hydrofluoric acid the entire ribbon and 

 teeth stain with borax carmine and other preparations, though 

 with differences in intensity in the various parts. When the 

 fresh radula is treated with the ordinary stains this is not 

 the case, as may be seen from fig. 8. The ribbon in this 

 figure has been stained first with Bethe's stain and afterwards 



