498 THE AMERICAN NATURALIST [Vol. LYI 



corals, typical inhabitants of the coastal waters, for the 

 sands and muds at greater depths offered few points of 

 attachment sufficiently stable. 



A Volvox-like colony of simple holozoic flagellates, ac- 

 cording to MacBride,^^ commenced to feed upon micro- 

 scopic organisms lying on the sea bottom, and under these 

 circumstances only the cells of the lower half of the 

 colony would be effective feeders. The upper cells, there- 

 fore, lost their flagella and became merely a protective 

 layer, which finally grew downwards outside the others 

 and fixed the colony to the ground. In this way a sponge 

 was formed. The collar cell, so typical of the group, had 

 been developed already by the flagellates, its first incep- 

 tion being perhaps a circle of protoplasmic hairs such as 

 we find in Pedinella. But this adoption of a fixed habit, 

 as it were mouth downwards, did not lead very far, and 

 though there has been much elaboration within the group 

 itself, the sponges have remained an isolated phylum, 

 unable to develop into higher forms. 



It is in a Ctenophore-like ancestor that we find the line 

 of development to higher animal groups, and this an- 

 cestor must have been at one time widely distributed in 

 the seas. Its immediate descendants are familiar to 

 every zoological student in the well-known series of pe- 

 lagic larval forms. Miiller's larva, taking to the bottom, 

 and in its hunt for food gliding over hard surfaces with 

 its cilia, led to the flatworms ; the PiUdium, developing a 

 thread-like body and creeping into cracks and crevices 

 to transfix its prey, gave rise to the nemertines. A Tro- 

 chophore, burrowing in soft mud and sand, developed a 

 segmented body which gave it later the power of running 

 on these soft surfaces, and became an annelid worm. An- 

 other Trochophore, developing a broad, muscular foot, 

 crept on the sand, and afterwards buried itself beneath 

 it as a lamellibranchiate mollusc, or migrated on to 

 harder surfaces as the gastropod and its allies. Pluteus, 

 Bipinnaria, Auricularia, first fixing, as the crinoids still 

 do, and developing a radial symmetry, afterwards broke 

 free and wandered on the bottom as sea-urchin, star-fish 



19 "Textbooks of Embryology. Invertebrata. " London, 1914. 



