26 On Pterodina vaJvata. 



evanescent thinness that I was unable to hold it between two plates 

 of thin glass without driving the drop of water away from the 

 rotifer. I made use of each of Eoss' later compressoriums, and 

 it is the first time that they have ever failed me ; their admirable 

 construction has hitherto enabled me to hold gently the minuter 

 rotifers in the smallest possible quantity of water. The advantage 

 of being able to bring two plates of glass together, within the 

 distance of a rotifer's thickness, with a truly parallel motion, is this ; 

 that the drop of water in which the rotifer is placed may be made 

 as small as the rotifer itself will bear without fear of the fluid's 

 being drawn away from it altogether by the want of parallelism of 

 the plates ; for these, when inclined to one another at a very minute 

 distance, act as a capillary tube, and whisk off the water in the 

 direction towards which they incline. 



" But," it may be asked, " why place the rotifer in a drop scarcely 

 bigger than itseK ? " This is done in order that the animal may be 

 placed, once for all, without chance of escape, in nearly the exact 

 centre of the compressorium, so that all the apparatus under the 

 stage may be brought, if need be, to bear upon it, without risk of 

 being impeded by the compressorium itself. 



Moreover, when the rotifer has been so placed, if the observer 

 gets tired, or the rotifer seems distressed by having been held for 

 some time, the slightest slackening of the screw gives the creature 

 a watery cage to swim in so small that it remains safely within the 

 field of the objective. 



If a single Pterodina happens to be placed with Algae in the 

 live cage, nothing can be more charming than to watch it under 

 dark field illumination with a low power, but little can be learned 

 of its structure. The thickness of the Algse secures for it watery 



EXPLANATION OF PLATE LXXIL 



Fig. 1 . — Under-surface of young female of Pterodina valvata. 

 m, mastax. 

 p, p, pear-shaped glands below mastax, and adhering to cesophagus. 



q, similar glands, where the oesophagus enters the stomach. 

 g, g, gastric glands. 

 s, stomach. 

 0, ovary. 

 V, V, V, vibratile tags. 



/, I, longitudinal striated muscles. 

 t, t, transverse muscles for folding lorica. 

 «, n, transverse muscles for closing the slit in the neck. 

 c, c, dilated ends of canals of water vascular system. 

 „ 2. — Side view of P. valvata. 

 „ 3. — Dorsal view of the same, with lorica folded, 

 „ 4. — Side view of the same ; lorica folded. 



„ 5. — Enlarged view of the under-surface of the front of the lorica — the animal 

 withdrawn to show the flaps which are closed by the muscles n, n, 

 Fig. 1. 

 „ 6.— Trochal disk, viewed from above, and showing the two eyes. 



