132 THE MICROSCOPE 



creature, lias also a large disc-like surface where there is prac- 

 tically no sldii to its jelly. The cilia which vibrate around this 

 disc cause a powerful current to flow, and it is amazing to watch 

 the smaller kinds of Protozoa being hustled into the creature's 

 body, where they swim about for a second or two and are then 

 still. 



As a final stage we find creatures with only one small open- 

 ing where food can be absorbed, and the complete development 

 of the mouth is here concluded. 



One of the earlier figures shows the dainty little Collared 

 Monad, which consists of a single cell with a vibrating whip, or 

 flageUum, and a very perfect little cup or collar of transparent 

 material. They are often found in large colonies on the surface 

 of weeds. To them we owe our sponges. A microscopic examina- 

 tion of one of the holes of a growing sponge reveals a colony of 

 these little organisms, closely arranged aU round the interior 

 of its surface. These have the power of creating instead of 

 shells a fibrous material, which forms 

 the matrix in which they are embedded. 

 In place of shells they deposit hard, flinty 

 spines in the substance of this matrix, 

 and these are called spicules. Thus a 

 sponge consists of myriads of colonies of 

 Collared Monads, their vibrating flageUse 

 causing a current of water to rush through 

 every cavity of the entire sponge, in order 

 to provide the food and oxygen necessary 

 Fig. 128. — Sponge. for the support of the community. 



The Protozoa show in a series of inter- 

 esting stages the gradual development of creatures of one cell. 

 Each ceE is complete in itself, though, as in the case of the 

 sponge, an approach to a more elaborate form is seen. Never- 

 theless, here each organ eats for itself, breathes for itself, re- 

 produces itself by splitting in half, and is an individual. 



Later stages of development show creatures of more than 

 one cell, in which some cells perform one function and some 

 another, and none are complete by themselves ; and the develop- 

 ment of the simplest form of life into a more complex animalcule 

 as indicated by a study of the Protozoa is but an indication of 

 the interest that can be obtained by the use of the microscope. 



The more elaborate and highly organised creatures met with 

 in water have equal charm and variety. The manner in which 

 they feed upon each other, the manner in which some become 

 parasites, and the methods of reproduction, are all subjects which 

 well repay investigation. The development of many of the animal- 

 cula from the egg to the finished and perfect creature has a special 

 fascination, because naturalists have discovered that in this 

 change from stage to stage which certain forms go through 



