126 THE HISTORY OF CREATION. 
of movements like those of an Amceba. The Planza, 
on the other hand, consisted of two kinds of different 
cells—inner ones like the Amcebee, and external “ ciliated 
cells.” By the vibrating movements of the cilia the entire 
multicellular body acquired a more rapid and stronger 
motion, and passed over from the creeping to the swim- 
ming mode of locomotion. In exactly the same manner 
the Morula, in the ontogenesis of lower animals, still 
changes into a ciliated form of larva, which has been 
known, since the year 1847, under the name of Planula. 
This Planula is sometimes a globular, sometimes an oval 
body, which swims about in the water by means of a 
vibrating movement; the fringed (ciliated) and smaller cells 
of the surface differ from the larger inner cells, which 
are unfringed. (Fig. 4 of the Frontispiece.) 
Out of this Planula, or fringed larva, there then develops, 
in animals of all tribes, an exceedingly important and 
interesting animal form, which, in my Monograph of the 
Caleareous Sponges, I have named Gastrula (that is, larva 
with a stomach or intestine). (Frontispiece, Fig. 5,6). This 
Gastrula externally resembles the Planula, but differs es- 
sentially from it in the fact that it encloses a cavity which 
opens to the outside by a mouth. The cavity is the “pri- 
mary intestine,” or “primary stomach,” the progaster, the 
first beginning of the alimentary canal; its opening is the 
“primary mouth” (prostoma). The wall of the progaster 
consists of two layers of cells: an outer layer of smaller 
ciliated cells (outer skin, or ectoderm), and of an inner 
layer of larger non-ciliated cells (inner skin, or entoderm). 
This exceedingly important larval form, the “ Gastrula,” 
makes its appearance in the ontogenesis of all tribes of 
