Dec. i895. FORMALIN AS A PRESERVATIVE. 417 



Protozoa. — Perfect preservation, with cilia distinctly shown, is 

 attained in a six per cent, solution. For microscopical fluid-mounts of 

 these organisms, carbolised water and allied fluids may be 

 advantageously discarded for this new medium, and by the use of 

 aqueous staining fluids containing a percentage of formalin, good 

 differentiation can be obtained ; weak bismarck brown is one of the 

 best stains to employ. 



PoRiFERA. — For histological details, formalin preparations are 

 inferior to such as have been thoroughly well fixed and passed 

 carefully into spirit, but the former are infinitely superior to such spirit 

 specimens as have not had the utmost care bestowed upon them. The 

 same observation applies fairly universally to other groups. 



For ordinary dissection, formalin preparations are equal to the 

 most carefully prepared spirit ones; while for museum preparations, 

 those in formalin are greatly superior, showing absolutely no con- 

 traction, and in those where delicate filmy tissue is present the lovely 

 transparency of life is retained almost unimpaired. 



HvDROiDEA. — For microscopical manipulation, spirit is superior, 

 as, when staining is employed after subsequent grading into alcohol, 

 there is found to be a lack of differentiation not present in specimens 

 prepared in the ordinary way. As a killing agent, I have, however, 

 successfully em.ployed formalin upon the Gymnoblastic Hydroids, 

 and find that imperfect staining can be obviated by the immediate 

 transference of the animals, after death in the formalin, to an 

 ordinary fixative, with subsequent grading into spirit. 



For dissecting and museum preparations, formalin is much more 

 satisfactory than spirit, giving exquisite transparency, very marked 

 in contrast with the dull opacity of spirit-preserved specimens. 

 Specially fine are the fleshy medusae of the Savsia type, preserved by 

 simple transference, while alive, into the fluid. 



For the Calyptoblastic Hydroids, formalin has no special 

 advantage. 



AcTiNozoA. — These, as a whole, give excellent results in formalin, 

 provided they be kept in not less than six per cent, strength. For those 

 with very thin- walled tentacles, e.g., Tealia crassicornis, a seven per cent, 

 or even an eight per cent, solution is preferable ; preparatory 

 hardening in corrosive sublimate is also beneficial, but this some- 

 what impairs the transparency. 



DiscoMEDus^. — The larger medusae have long been considered 

 well-nigh the most difficult of organisms to preserve satisfactorily. 

 By the employment of formalin, all difficulty disappears, as after a 

 number of experiments this summer I discovered that simple 

 immersion in a five per cent, solution is all that is necessary. To-day, 

 after the lapse of six months, the specimens are as good as, and indeed 

 considerably tougher than, the first day they were placed in the medium. 

 They {Aurelia and Rhizostoma) are glassy transparent, with the radial 

 canals showing conspicuously in a delicate opalescent white. 



2 G 



