338 MICROSCOPICAL TECHNIQUE. 



in which they had been allowed to expand, a saturated solution of 

 MgSO^ (Epsom salts). 



Verj7ies. — Most annelids, when placed in saturated solution of 

 Epsom salts, in a very short time became perfectly limp, and were 

 easily extended upon a glass plate and treated with a fixing 

 reagent. Bala?ioglossus, when taken soon after being collected, 

 was preserved in this manner in nearly a perfect state. It was 

 necessary, however, to keep it in position between the edges of 

 two glass slides when the fixing fluid was applied. Good results 

 were obtained with Cirratulus^ Amphitrite^ Nereis^ R/iy?icobolus, 

 Clymenella, and Phascolosoma. Fhascolosonia, in most cases, was 

 killed with tentacles protruded. Nemertean worms when trans- 

 ferred to a killing fluid before being completely narcotised, some- 

 times protruded their probosces. 



Ascidiaiis. — Molgida and Cynthia were readily killed with 

 siphons open after ansesthetisation with magnesium sulphate. In 

 this case it is best to add the saturated solution of sulphate 

 intermittently with a pipette. 



Ctenophores. — After considerable experimentation, a method for 

 preserving these delicate creatures in a nearly life-like appearance 

 was devised. Formalin alone in solutions of varying strength had 

 been tried without success. It was found necessary to treat the 

 animals with some hardening re-agent before placing them in the 

 formalin, and the following method seems to be the most success- 

 ful: — To a solution of equal parts of 2 per cent, formalin and 

 Perenyi's fluid was added enough common salt (NaCl) to increase 

 the density of the mixture to that of sea-water — /.(?., until a Cteno- 

 phore placed in it barely floated. This adjustment of the density 

 of the surrounding medium prevented the Ctenophores from 

 collapsing of their own weight. After remaining for about half-an- 

 hour in this fluid, they were transferred to 4 per cent, formalin, the 

 density of which had been increased by the addition of either 

 Epsom salts or common salt, so that the Ctenophores again barely 

 floated. Epsom salts is probably better than common salt for 

 increasing the density of the fluid. Some specimens which were 

 preserved in formalin -I- NaCl began to shrink after a few days ; 

 while some {M?7e>niopsis) which have been preserved for nearly six 

 months in formalin + MgSO^ are still in excellent condition. 



