PROCEEDINGS OF THE SOCIETY. 121 



about the same density as the water in which the creature lives. The 

 integument is a thin membrane, which allows an exchange of fluid only 

 by osmosis. When a Rotifer — which has been duly narcotised, fixed, and 

 killed, fully extended with osmic acid or other fixing agent — is placed in 

 say, a 5 p.c. watery solution of glycerin ( having a specific gravity of about 

 1-017), the fluid within the body-cavity will pass out quicker than the 

 denser outer fluid can flow in, and the result is a shrinkage and often 

 complete collapse of the Rotifer. By beginning with a very weak solu- 

 tion of glycerin and increasing this h p.c at a time, I have with infinite 

 trouble succeeded in about three weeks in mounting some Euchlanis and 

 other Rotifers in pure glycerin, but the advantages gained were hardly 

 worth the expenditure of time involved. When Rotifers are to be 

 mounted in balsam, the trouble begins with the alcohol ; then there is 

 the passage through oil and balsam, each denser than the preceding fluid. 

 The method is therefore a work of very considerable difficulty, skill, and 

 patience, and Mr. Tozer must be congratulated on accomplishing so much. 

 A minor drawback is the necessity of deeply staining the Rotifers, other- 

 wise the balsam will render their tissues and organs so transparent 

 as to be scarcely visible. 



The mounting of Infusoria in Canada balsam does not present the 

 same difficulties, because Infusoria have no proper body-cavity, and 

 their bodies consist wholly of protoplasm, which can be fixed and 

 hardened by well-known reagents to any desired degree. 



Of previous successful attempts to mount Rotifera in balsam, may 

 be mentioned that of N. de Zograf, who fixed the animals with osmic 

 acid and raw wood vinegar.* 



Quite recently I received a few Eosphora quite well mounted in balsam 

 from a correspondent in Germany, who fixed the Rotifers with mixtures 

 of picric acid and acetic acid, and picric acid and chromic acid, and, 

 after passing through alcohol, adding graduated mixtures of absolute 

 alcohol and clove oil, and finally balsam. 



Of the advantage, for permanent keeping, of mounting Rotifers 

 in balsam there can be no doubt, but it seems evident that all the 

 methods of doing so will always be very tedious, and involve much ex- 

 penditure of time. The Society, however, must be grateful to Mr. 

 Tozer for bringing this subject before them, and describing his method 

 of procedure and exhibiting the results he has achieved. 



Mr. Wesche said that he had been rather surprised, on examining 

 Mr. Tozer's preparations, to see how good the results were, although he 

 was still of the opinion that the only really good Rotifer was a live Rotifer. 

 But whatever the process, a permanent" record was valuable, and even 

 necessarv, to enable the naturalist to differentiate between the species of 

 difficult * genera, As an illustration of the permanence of such mounts, 

 he had some slides of the aquatic larvae and pupa of gnats (Cidex) which 

 had been mounted in Canada balsam forty years ago. One of these slides 

 had been accidentally damaged, and the balsam in the centre was still in a 

 fluid state. 



The Rev. Eustace Tozer said there was a reason for using oil of 



* Described by birn in Comptes Rendus, exxiv. (1897) pp. 245-6. For exi 

 see tbis Journal, 1897, p. 173. 



