Natural History of the Animalcules. 107 
parable to the Animalcules, consists of a compound body 
comparable to the Zoophytes, made up of a dense aggre- 
gation ot buds around a multicellular parent form, in 
which the cells are arranged in at least two definite 
layers — an outer sensory, and an inner digestive layer — 
from the outer of which a nervous system of a simple 
type is developed. The term, also, must be considered ex- 
clusive of the highly-organised, free-swimming, larval stages 
which are characteristic of many of the higher animals, 
notably of the worms, molluscs, etc.; while its wide appli- 
cation, in a popular sense, to include undoubted vegetable 
forms, is so evidently inaccurate that it needs no remark. 
Animalcules are found throughout all parts of the 
globe, in fresh and salt water, and in situations that are 
more or less permanently moist and washed by water. 
The sea may be described as their great home, but 
ponds, pools, lakes, trenches and streams, especially 
those that are stagnant or contain a large amount 
of decaying matter, furnish an inexhaustible supply. 
So far we have been concerned with the question of the 
general nature and strufture of the Animalcules, pointing 
out in what way, and for what reasons, the name should 
be limited or extended from its former and popular 
signification. From this standpoint it will be of some 
advantage to pass in quick review a few T of the more 
typical Animalcules ; for, in this way, not only shall we 
become familiarised with models, by means of which we 
shall be enabled to design accurate pictures of the group, 
but we shall also obtain some idea of the chief orders or 
grades into which this multiform assemblage is divided. 
For this purpose the models will be sele6led from those 
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