clxiv Introduction. 



by the aid of which it swims about ; at another it may have lost its 

 cilium, and effect locomotion by the protrusion of pseudopodia, like an 

 Amoeba ; whilst in a third it may surround itself with an envelope of 

 cellulose. If it should prove to be true that organisms as high in the 

 scale as the Amoehina and Actinophryna, can have their development 

 traced back to the specialization of protoplasm within vegetable cells, 

 it would appear to be necessary to adopt a phraseology which should 

 speak of such creatures as being at one time plants, and at another 

 animals. 



Class, Infusoria. 



Protozoa, which have the external layers of their bodies so far 

 indurated as to give them more definitely-fixed external outlines 

 than the other classes of the Sub-king-dom, and are provided, except 

 in the case of the adult Acinetina, with cilia as motor org-aus. They 

 may secrete a cuticular shell or carapace, distinct from their more or 

 less indurated external cortical cuticular envelope, but they never 

 form any shell or skeleton of inorganic substances. They always 

 possess a nucleus and nucleolus, which are in function ovary and 

 testis respectively, and a contractile vesicle ; and with the exception 

 of the order Acinetina, which is provided with suctorial tentaeula, 

 and the parasitic genus Opalina, they have always a mouth and 

 anus. From the mouth a short oesophagus lined with a prolonga- 

 tion of the cuticle leads ordinarily, and opens by an oblique or 

 transverse aperture, into the central parenchyma of the body, which 

 is more loosely compacted than the cortical layers. In the inter- 

 trabecular or vacuolated spaces of this central parenchyma, the 

 ingested alimentary particles are circulated together with the water, 

 along with which they are drawn in by the ciliary currents ; and 

 from it the refuse particles are finally extruded by the anus, into 

 which there is only rarely a tubular process of the cuticle prolonged. 

 The cortical layers of the parenchyma are less diffluent than the 

 central, and in them we find the contractile vesicle, the generative 

 nucleus with its adherent or closely approximated nucleolus, the 

 triehocysts, and a certain amount of very fine pale granular matter. 

 By the use of reagents, it is easy to see that the cilia are in reality 

 processes not of the cuticular membrane, but of the outer layers of 

 the enclosed parenchyma ; and they must therefore when protruded 

 find their way through very fine orifices in the external envelope. 



