NATURAL HISTORY OF BHOMPSTON POND. 203 



of the cilium is due to an injection of hyaloplasm from the 

 cell. A typical example of ciliata is Opalina ranorum (fig. 3), 

 a parasitic form covered with cilia (a) of equal length 

 (Holotricha). It occurred in hundreds in the intestine of a 

 frog and exhibited ciliary movement more clearly than any 

 other form I have seen. This animal has no mouth, as it 

 absorbs food through any part of the body. Fixing and 

 staining revealed a large number of nuclei (b). A common 

 free-living form is Paramcecium (fig. 4), an ovate oblong form, 

 covered with cilia of equal length excepting in the pharynx, 

 where they are longer. In the ectosarc (a) is a layer of oval 

 trichocysts, the occasionally discharged threads (f) of which 

 can be detected with the high power ; after fixing and staining, 

 a micronucleus as well as a meganucleus (g) can be detected. 

 Towards the anterior and posterior ends of the body one can 

 observe the changes associated with the contractile vacuoles. 

 If watched, the spherical vacuole (b) suddenly vanishes from 

 view, and shortly after its place is occupied by five or six 

 isolated drops, each with a minute streak passing into it (c). 

 At first the drops have a star-like arrangement, but on 

 increasing in size they coalesce to form the contractile vacuole. 

 When a little Indian ink was irrigated under the cover-glass, 

 not only were the ciliary movements more clearly seen, but 

 the process of food ingestion accurately observed. The 

 minute particles of the ink are collected at the end of the 

 pharynx (d),and when a sufficient quantity has accumulated 

 the little mass is ingested, together with a small quantity of 

 water forming a food- vacuole (e), which can be seen to pass 

 through the body, the residue being ejected at a definite spot. 

 Paramoecium occurs plentifully in most parts of the pond. 



An allied animal, Trachelocerca (" Swan-neck animalcule ") 

 (fig. 5), seems scarce in this pond, only a few specimens being 

 met with in hundreds of pond samples. It is a most curious 

 fact that these animals, microscopic as they are, frequently 

 hunt for their food in pairs. The ciliated transparent body 

 (a) tapers to a short tail (b) at one end, and at the other is 

 continued as the long slender, so-called neck (c), which 



