276 HAECKEL 



imagine it swimming ahead in the water or 

 creeping along the ground in such a way as to 

 assume a bilateral symmetrical structure, like a 

 tube, with right and left, back and belly, and 

 an anus behind, we have a worm. This worm 

 developed, under the action of the Darwinian 

 laws, into a star-fish in one case, a crab or insect 

 in another, a snail or mussel in another, and 

 lastly into the amphioxus, which led on through 

 the vertebrates to the human frame. But the 

 mysterious series of forms always remained in 

 the development of the individual from the egg, 

 pointing more or less clearly to the earlier stages : 

 ovum, cluster of cells, ball, two cell-layers in a 

 cup-shaped form, skin, stomach, and mouth. All 

 animals that exhibit this primitive scheme belong 

 to one great stem. It was not until this skin- 

 stomach-mouth animal was formed that the tree 

 branched out evolving into sessile, creeping, 

 swimming, and other forms. Let us give a name 

 to this phylogenetic (ancestral) form, which stands 

 at the great parting of the ways in the animal 

 world, as embryology proves. Leaving aside its 

 innumerable relatives in the primitive days, it 

 must have differed essentially from all other living 

 things at the time all the protists and the plants 

 by its possession of a skin, stomach, and mouth. 

 Gaster is the Greek for stomach. Let us, there- 

 fore, call this primitive parent of all the sponges 

 polyps, medusae, worms, Crustacea, insects, snails, 

 mussels, cephalopods, fishes, salamanders, lizards, 

 birds, mammals, and man, the gastraa, the primi- 



