126 THE HISTORY OF CREATION. 



of movements like those of an Amoeba. The Plansea, 

 on the other hand, consisted of two kinds of different 

 cells — inner ones like the Amoebse, 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 lai-va, 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 aU tribes, an exceedingly important and 

 interesting animal form, which, in my Monograph of the 

 Calcareous 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- 

 sentiallv from it in the fact that it encloses a cavity which 

 opens to the outside by a mouth. The cavity is the " pri- 

 raary intestine" or "primary stomach," the progaster, the 

 first beginning of the alimentary canal; its opening is the 

 "primary movdh" (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 aU tribes of 



