STRUCTURE OF I HE INFUSORIA. 



355 



Fig. 3. CEBCOMOXAS TYPICA. (Saville Kent.) 



A, Adult; u l, different stage? in the development. (After 

 Dalliiiger ana Drysdale.) 



Pond water and artificial infusions of hay yield, as a rule, considerable numbers of a rather large 

 animalcule, which may be from ^th to T ?roth of an inch in length. They are free swimmers and long- 

 bodied, being narrowish and bluntly pointed at one end, and more sharply at the other. They are flat 

 also, and there is a groove in the body extending from the left side of the front part of the body back- 

 ward and underneath to about the middle. They are about four times as long as broad, and their shape 

 has given them the name of Slipper animalcules (Fig. 2).* They are not quite symmetrical fore-and-aft, 

 and the back and ventral surface can be distinguished. The whole of the body is covered with a fine 

 down of cilia of nearly or quite equal size throughout, which vibrate with considerable rapidity, 

 enabling the animal to move here and there rapidly, to turn round on its axis, to swim backwards 

 and forwards, and even to turn like a screw on its long axis, throwing the under part up and over, to 

 replace the back in its original position. As these animalcules, which have a yellowish-brown tint 

 by transmitted light, move vigorously along, they rush over the field of the microscope and re-enter, 

 and should there be a collection of vegetable mucus, numbers will come together and push in and 

 amongst it, passing here and there, but never brushing 

 up against one another, so as to come into collision. It 

 is evident that they have some power of slightly altering 

 the shape of the body, and that the slit on the underside 

 has to do with the inception of food. The cilia, when the 

 animal is moving or comparatively still, form currents in 

 the water, and those in the neighbourhood of the slit 

 produce whirlpools, down which rush minute particles of 

 food. These pass down the slit, and enter the body at 

 a kind of mouth, and they there come in contact with 

 the soft inner substance composing the animalcule, and 

 sink into it, being surrounded by a drop of water. 

 Several of these morsels of food are to be seen lying 

 in clear spaces filled with water or food vacuoles, and as the whole of the soft internal structure 

 tends to move in an amoeboid kind of manner, the vacuoles change their places. This gaA'e rise 

 to the false idea that the Infusoria were many-stomached, or " polygastrica." In this internal 

 substance, or endoplasm, some other things are to be seen. Firstly, there is an oval body 

 with a small dark spot in it, the nucleus or endoplast, and the nucleolus or endoplastule ; secondly, 

 there are two spots, one close to either end of the body, which gradually become more visible and 

 transparent, and suddenly shut up and disappear. They are the contractile vesicles, and it is 

 commonly observed that, if the animalcule is subjected to any pressure, these light spots present 

 rays passing from them into the endoplasm, so as to assume a stellate appearance. The opening and 

 closing of these vesicles are very regular. There is a most delicate tissue covering the whole animalcule, 

 and another from which the cilia spring. They are elastic, and appear to be endoplasm in a less 

 watery condition. They form the ectoplasm. Between these layers and the minutely-granular 

 endoplasm is one of exceedingly delicate rod-like bodies arranged point outwards, and they are called 

 trichocysts. 



The animalcule evidently respires through its outer ciliated coat, takes in food through the mouth 

 at the bottom of the slit, has several food vacuoles, which finally come near the surface skin, and dis- 

 chai-ge the undigested matters. As the food, consisting of minute spores and animal and vegetable 

 matters, is digested, the protoplasm of the body is added to, and the circulation and removal of effete 

 matters are in relation to the contractile vesicles. 



The creatures languish if the water remains too long without exposure to air, but otherwise their 

 movement appears to be constant. Occasionally two will approach and cling together by their oral 

 or ventral surfaces, and it is occasionally noticed that a large individual contracts midway and finally 

 separates into two. If watch be kept long enough, the animalcules will be noticed to become quiet, 

 to take on a globular form, and to have the ectoplasm dense and non-ciliated. Sooner or later the 

 globe will burst, and a host of minute moving things will come forth, each of which is a young 

 animalcule. 



Paramechnit aurelia. 



