872 SPONGES AND ZOOPHYTES 



the syringe ; and it is well to mount specimens in several dif- 

 ferent menstrua, marking the nature and strength of each, as 

 some forms are better preserved by one and some by another. 1 

 An excellent method of preservation has been discovered by 

 M. Foettinger 2 in the use of chloral hydrate : when all the 

 polypes in a vessel containing 100 c.c. of water are fully expanded 

 some crystals of chloral hydrate are to be dropped into the 

 vessel ; these dissolve rapidly and gradually diffuse through the 

 water. About ten minutes later a little more chloral should be 

 added, and in three-quarters of an hour the whole colony will be 

 found to have become insensible ; the advantage of this method 

 lies in the fact that the action is merely narcotic, and .that the tissues 

 are not affected. When the influence is so complete that irritation 

 fails to produce retraction of the polypes the colony may be put into 

 alcohol. The size of the cell must of course be proportioned to that 

 of the object ; and if it be desired to mount such a specimen as may 

 serve for a characteristic illustration of the mode of growth of the 

 species it represents, the large shallow cells, whose walls are made 

 by cementing four strips of glass to the plate that forms the bottom, 

 will generally be found preferable. The horny polyparies of the 

 Sertulariida, when mounted in Canada balsam, are beautiful objects 

 for the polariscope ; but in order to prepare them successfully some 

 nicety of management is required. The following are the outlines 

 of the method recommended by Dr. Golding Bird, who very success- 

 fully practised it. The specimens selected, which should not exceed 

 two inches in length, are first to be submitted, while immersed in 

 water of 120, to the vacuum of an air-pump. The ebullition 

 which will take place within the cavities will have the effect of free- 

 ing the polyparies from dead polypes and other animal matter ; and 

 this cleansing process should be repeated several times. The 

 specimens are then to be dried, by first draining them for a few 

 seconds on bibulous paper, and then by submitting them to the 

 vacuum of an air-pump, within a thick earthenware ointment-pot 

 fitted with a cover, which has been previously heated to about 200 ; 

 by this means the specimens are very quickly and completely dried, 

 the water being evaporated so quickly that the cells and tubes 

 hardly collapse or wrinkle. The specimens are then placed in 

 camphine, and again subjected to the exhausting process for the 

 displacement of the air by that liquid ; and when they have been 

 thoroughly saturated, they should be mounted in Canada balsam in 

 the usual mode. When thus prepared they become very beautiful 

 transparent objects for low magnifying powers ; and they present a 

 gorgeous display of colours when examined by polarised light, with 

 the interposition of a plate of selenite, the effect being much en 

 hanced by the use of black-ground illumination. 



No result of microscopic research was more unexpected than 

 the discovery of the close relationship subsisting between the 

 hydroid Zoophytes and the medusoid Acalephce (or 'jelly-fish '). We 

 now know that the small free-swimming medusoids belonging to 



1 See Mr. J. W. Morris in Quart. Journ. of Microsc. Sci. n.s. vol. ii. 1862, p. 116. 



2 Archives de Biologie, vi. p. 115. 



