MOUNTING ZOOPHYTES FOR POLARIZATION. 543 



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 (§ 170), will generally 

 be found preferable. 



420. The horny Polyparies of the Sertularidse, 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 recom- 

 mended by Dr. Golding Bird, who very successfully 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 freeing the 

 Polyparies from dead Polypes and other animal matter ; and this 

 cleansing process should be repeated several times. The speci- 

 mens 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 trans- 

 parent objects for low magnifying powers ; and they present a 

 gorgeous display of colours when examined by Polarized light, 

 with the interposition of a plate of Selenite. These objects are 

 peculiarly fitted for the use of the Polarizing Apparatus in combi- 

 nation with the Spot-Lens (§ 88) ; as they then exhibit all the 

 richness of coloration which the former developes, with the pecu- 

 liar solidity or appearance of projection which they derive from 

 the use of the latter. 



421. No result of Microscopic research has been more unexpected, 

 than the discovery of the close relationship subsisting between the 

 Hydroid Zoophytes and theMedusoid Acalephce (or Jelly-fish.) We 

 have seen that many of the small free-swimming Medusoids, belonging 

 to that simple tribe of which Thumantias (Fig. 278) maybe taken 

 as a representative, are really to be considered as the detached 

 Sexual apparatus of the Zoophytes from which they have been 

 budded-off, endowed with independent organs of Nutrition and 

 Locomotion, whereby they become capable of maintaining their 

 own existence and of developing their Generative products. The 

 general conformation of these organs will be understood from the 

 accompanying figure. Many of this group are very beautiful objects 

 for Microscopic examination, being small enough to be viewed en- 

 tire in the Zoophyte-trough. There are some few parts of the coast 



