530 METHODS FOR INVERTEBRATES. 



or into one of the specimen tubes to carry home. The attached forms, 

 such as hydra, polyzoa, vorticella, are generally found on water weeds 

 or bits of stick or submerged roots. In order to secure such organisms 

 the weeds, etc., are severed with the cutting knife and are dragged on 

 shore,' placed in water in the flat bottle and examined. Polyzoa seem 

 to have a preference for submerged rootlets, whilst forms such as 

 vorticella are more frequently found on the roots and stems of duckweed. 



998. Determination of Life Cycles. The only really satisfactory 

 method of determining life cycles is to follow a single organism con- 

 tinuously throughout its various changes in the manner originally used 

 by the late Dr. Dallinger, but such observations should be controlled 

 by examination of suitably stained specimens in the different stages, 

 under critical illumination. In the case of many of the parasitic pro. 

 tozoa such methods are impossible, as cultures cannot be obtained. In 

 such cases we are only able to examine fixed and stained preparations 

 and endeavour to piece out a life cycle from the appearances observed. 

 This must be checked by observations of the living material wherever 

 possible. For free living protozoa some means of keeping a drop of 

 the culture fluid from drying is necessary, but any means adopted must 

 permit of the continuous examination of the organisms by high -power 

 lenses. One of the best is that used by Dallinger and Drysdale, and 

 described in The Microscope and its Revelations, edited by DALLINGER, 

 8th ed., Part I, pp. 341 to 344. For the majority of flagellates this is an 

 excellent arrangement. Its chief drawback seems to be due to the fact 

 that it does not permit the aeration of the culture fluid, and it is, there- 

 fore, found that organisms such as large ciliates, many amoebae, vorti- 

 cellse, etc., soon die out from lack of oxygen. The simplest method of 

 examining such forms is the well-known hanging drop arranged on a 

 hollow ground slide. This, however, does not permit of critical illumina- 

 tion and completely upsets the corrections of the condenser. To get 

 over this difficulty a slide with a small table ground out is used. The 

 table is surrounded by a trench, and a ring of vaseline is painted round 

 the outside of the trench, and a drop of the fluid containing the 

 organisms to be studied is placed on the table. A cover -glass is then 

 lowered on to the drop and adjusted for pressure by gently pressing 

 on to .the vaseline. It is advisable in most cases to arrange the drop 

 so that it does not spread entirely to the edge of the table ; this ensures 

 an air supply. For amoebae and many ciliates and flagellates the live 

 slide described by DREW and GRIFFIN (Journal of Royal Microscopical 

 Society, February 21st, 1917) may be used. This form consists of a glass 

 plate cut to fit the mechanical stage of the microscope, and with a glass 

 arm cemented along one side. A piece of linen has a hole slightly 

 larger than the cover- glasses to be used cut in it, and this is then 

 damped with water and laid upon the slide. A small glass vessel filled 

 with water is attached to the arm and is put in communication with the 

 cloth by means of a piece of linen or soft wick. The linen is thus kept 

 moist by capillarity. A drop of the culture is placed on the slide and a 

 cover-glass placed upon it, and adjusted so that the circular opening in 

 the linen touches it at the margins, pressure being regulated, if necessary, 



