106 TlMEHRt. 
table substances. But this is essentially an unwise 
limitation, since all the minute and simple forms 
are properly Animalcules — all those, in fa6l, which, 
while at the same time minute, possess that simplicity of 
stru6lure that points to a close or considerable degree of 
relationship, as compared with the higher forms. By 
this same principle, the term must be held to be properly 
exclusive of the group of minute worms or worm-like 
forms which are popularly called " Wheel-animalcules," 
and technically Rotifera i both names referring to the 
circular or disc-shaped anterior portion of their body, 
which portion, being lined with rapidly vibratile cilia, 
gives rise to an apparent, though not real, rotatory 
motion of the organism through the water. These forms, 
though minute, possess a high degree of organisation, 
evidenced by their distin6l muscular, nervous, repro- 
ductive, digestive and excretory systems, which are of 
such a kind as to ally them to the groups of the worms 
and other segmented animals — far removed from the 
simple organisms with which the older naturalists united 
them, and with which the only common feature, in the 
light of modern knowledge, is their similarity of size and 
motion. 
Until quite recently, the Animalcules also inclu- 
ded the multitudinous minute beings that were consi- 
dered to make up the composite body of the sponges — 
an application of the term that was striftly accurate in 
the light of the knowledge of that time. Owing to the 
knowledge that we at present possess as to the constitu- 
tion of sponges, however, such an application is no 
longer permissible ; for the sponge-body, instead of 
being a colony of structurally minute, simple beings com- 
