CLASSES OF EGG- ANIMALS. 135 



a thick, simple membrane, which surrounds their cell-body ; 

 they can be considered as animal Amoebae 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 

 xuching Infusoria (Acinetae), which take their food by means 

 (jf 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 Amoebae. 



The second main class of primaeval animals consists of the 

 Germ animals (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 ceUs, and 

 these cells moreover differentiated — in two ways at least — 

 into an external (animal or dermal) and an internal 



