14 



INTRODUCTION TO ZOOLOGY 



Structure. 



Cuhck 

 torfex 

 mcdullA 



-For full investigation of the structure of the 

 body, a compound microscope will be needed. 

 The Vorticella, still attached to the weed, should be 

 mounted in a drop of water, carefully covered with a cover- 

 slip, and examined under the low power of the microscope. 

 It will then be seen that though the body consists of one 

 single mass of protoplasm, this protoplasm is differentiated in 

 different parts of the body ; also it is surrounded externally 

 by a delicate, transparent, limiting layer, the cuticle, which 

 was not present in Amoeba. 



In the " bell " itself, the protoplasm is differentiated into 

 an outer, denser, cortical portion, 

 and a less dense central "medulla." 

 Lying in the medulla is a large 

 horse-shoe . shaped nucleus, the 

 macro-nucleus, and also a small 

 round nucleus, which seems speci- 

 ally concerned with the repro- 

 ductive processes (see page 17). 

 At the free end of the bell, the 

 protoplasm is thickened to form 

 a slight rim round the edge, and 

 within this, but separated from 

 it by a depression except on 

 one side, the protoplasm rises 

 obliquely, forming the surface plate known as the disc (see 

 Fig. 5). This disc looks rather like a lid hinged at one 

 side, but it is really continuous with the protoplasm , of 

 the rest of the body. Just at the highest point of the disc, 

 there is, between it and the projecting rim of the bell, a 

 deep tubular depression, which runs right down into the 

 centre of the bell ; through this, food is passed into the 

 protoplasm, and hence it is called the gullet, and the open 

 upper end of it is the mouth. 

 Nutrition '^^^ ^°°^ consists of minute particles of animal 



and vegetable matter, suspended in the surrounding 

 water ; it is driven into the mouth by the movement of the 

 cilia, which are attached in a continuous single row round 

 the margin of the disc, and pass from this, round into the 

 inner margin of the rim and down the gullet. The regular 

 lashing of this spiral of cilia causes a little whirlpool in the 



con: 



Fig. 5. — Optical Section of Vorti- 

 cella to illustrate the union 

 of Disc and Bell. 



