CLASSES OF EGG-ANIMALS, 135 
_ a thick, simple membrane, which surrounds their cell-body ; 
they can be considered as animal Amcebe which have 
adopted a parasitical mode of life, and in consequence have 
surrounded themselves with a secreted covering. . 
As a third class of egg animals, we adopt the real 
Infusoria (Infusoria), embracing those forms to which 
modern zoology almost universally limits this class of 
animals. The principal portion of them consists of the 
small ciliated Infusoria (Ciliata), which inhabit all the fresh 
and salt waters of the earth in great numbers, and which 
swim about by means of a delicate garb of vibratile fringes. 
A second and smaller division consists of the adherent 
sucking Infusoria (Acinetze), which take their food by means 
of fine sucking-tubes. Although during the last thirty 
years numerous and very careful investigations have been 
made on these small animalcules,——which are mostly in- 
visible to the naked eye,—still we are even now not very 
sure about their development and form-value. We do not 
even yet know whether the Infusoria are single or many- 
celled ; but as no investigator has as yet proved their body to 
be a combination of cells, we are, in the mean time, justified 
in considering them as single-celled, like the Gregarines and 
the Amcebe. 
The second main class of primeval animals consists of the 
Germ anvmals (Blastularia). This name we give to those 
extinct Protozoa which correspond to the two ontogenetic 
embryonic forms of the six higher animal tribes, namely, the 
Planula and the Gastrula. The body of these Blastularia, in a 
perfectly developed state, was composed of many cells, and 
these cells moreover differentiated—in two ways at least— 
into an external (animal or dermal) and an_ internal 
