SPONGES AND ZOOPHYTES. 131 



r, weak Spirit, diluted Glycerine, a mixture of Spirit and Glycerine 

 with Sea-water or any other menstruum, by means of the Syringe; and it 

 is well to mount specimens in several different menstrua, marking the 

 nature and strength of each, as some forms are better preserved by one 

 and some by another. * The size of the cell must of course be propor- 

 tioned to that of the object; and if it be desired to mount such a speci- 

 men 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 

 ( 174), will generally be found preferable. The horny polyparies of 

 the Sertularida, 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 successfully prac- 

 tised 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 freeing 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 sub- 

 mitting 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 com- 

 pletely 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 dis- 

 placement 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 colors when examined by 

 Polarized light, with the interposition of a plate 

 Selenite, the effect being much enhanced by 

 the use of Black-ground illumination. 



522. No result of Microscopic research was 

 more unexpected than the discovery of the close 

 relationship subsisting between the Hydroid 

 Zoophytes and the Medusoid Acaleplim (or ' jel- 

 ly-fish'). We now know that the small free- 

 swimming Medusoids belonging to the ' naked- 

 eyed' group, of which Thaumantias (Fig. 358) 

 may be taken as a representative, are really to 

 be considered as the detached sexual apparatus 

 of the Zoophytes from which they have been fS ,, 



budded-off, endowed with independent organs and^terminating in the marginal 

 of nutrition and locomotion, whereby they be- 

 come capable of maintaining their own existence and of developing their 

 sexual products. The general conformation of these organs will be under- 

 stood from the accompanying figure. Many of this group are very beautiful 

 objects for Microscopic examination, being small enough to be viewed 



1 See Mr. J. W. Morris in "Quart. Joum. of Microsc. Science," N.S., Vol. ii. 

 (1862), p. 116. 



