OF THE PROTOZOA. CILIATA. 371 



Confervce, and Algce, which are difFiised throughout, or float upon the surface, 

 or form a stratum at the bottom. 



The attached forms find appropriate habitats upon the stems of aquatic 

 plants, and very commonly upon the surface of various animals living in 

 water ; for instance, on the shells of Mollusca, such as the water- snails, and 

 on the surface of the Entomostraca. A few species find a suitable locality 

 within the interior of larger animals, of which, therefore, they are esteemed 

 the parasites, — a fact illustrated in the genus Bursar la. This subject of the 

 habitats of the several genera needs not here to be enlarged upon, since it 

 recurs again and again in the generic and specific descriptions of the systematic 

 division of our work. 



The Ciliata do not so frequently constitute the coloiuing ingredients in 

 water as do the Phytozoa. Nevertheless there are several species which 

 make their presence known by their colour, either when collected in a stratum 

 upon the surface of plants or of the water, or when generally diffused in a 

 small pool. Thus Stentor iDohjmoyylms and Vorticella chlorostigma coat the 

 stems of aquatic plants green, whilst several species of Vorticellina cover them 

 as with a bluish-milky film, and Stentor aureus with an orange -coloured in- 

 duvium. Bursaria vernalis, Traclielocerca viridis, Coleps viridis, Glaucoma 

 viridis, and Paramecium Chryscdis are found dispersed through the water — the 

 four fii'st imparting to it a green, the last a milky tint. The greenish masses 

 of Oplirydium versatile at times float on the surface, driven about by the 

 wind, and at others are attached to the tendi'ils of roots and to the stalks of 

 aquatic plants. 



The chstinct colours, such as green, yellowish-red, and orange-brown, are 

 in all cases, we believe, not essential to the animalcules exhibiting them, but 

 due to the food they swallow, and to its changes in course of digestion. 

 These changes, as affecting the colour, have been illustrated in a preceding page 

 (p. 310) in the instance of Bursaria vernalis, for which the Chilodon ornatus 

 might have been substituted. Moreover, in Nassula the. reddish-blue or violet 

 spots, conceived to be glands by Ehrenberg, are apparently the product of di- 

 gested Oscillatorice (p. 312). 



SuccEssioi^^ OF Species. — If a fluid containing Infusoria be examined from 

 time to time over a considerable period, it will be found that certain species 

 disappear, and are replaced by others not before found in it. This succession 

 of forms in the same liquid has been remarked from the earliest period of 

 microscopic research, and has been the fruitful source of the wildest theories 

 of the metamorphoses of Lifusoria. Succeeding animals have been forth- 

 with concluded to be the transformed states of previous ones, however wide 

 the dissimilarity between them : no intermediate phases or transitional changes 

 have been watched; but the conclusion that the one is derived from the 

 other, has been jumped at without reserve. Some theorists have even pro- 

 ceeded further, and, like linger, behoved in the transformation of vegetable 

 into animal hfe, or, like Laurent and Gros, have imagined the conversion of 

 mineral matter into organized animalcules, and these last into beings of stiU 

 higher position in the animal scale, such as Annelida and Crustacea. 



A partial explanation of the succession of animal forms in a collection of 

 water is to be found in the following facts : — 



First, no vessel of water of ordinary dimensions can be so thoroughly ex- 

 amined but that some animalcules may be overlooked ; the same accident will 

 happen stiU more frequently with their minuter germs or embrj'onic condi- 

 tions, or with their encysted state. The earliest phases, again, may, in their 

 transient form, verj^ nearly resemble certain known independent species, and 

 be readily mistaken for them, or even for encysted simple plants. So, also, por- 



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