640 THE AMERICAN NATURALIST. (VoL. XXXII. 
The time which elapsed between the end of the Laurentian 
and beginning of the Cambrian was immense, or, at least, as 
long as the entire Paleozoic era. Walcott estimates the length 
of the Algonkian at 17,500,000 years. This length of time, or 
even a portion of it, was long enough for the origination and 
establishment of those classes, whose highly specialized descend- 
ants flourished in the Cambrian. Referring to the Precambrian 
strata Walcott states : 
That the life in the pre-Olenellus seas was large and varied there can be 
little if any doubt. The few traces known of it prove little of its character, 
but they prove that life existed in a period far preceding Lower Cambrian 
time, and they foster the hope that it is only a question of search and 
favorable conditions to discover it.! 
Here the imagination of the zodlogist may be allowed for the 
moment free scope to act. It is perhaps not hazardous to 
surmise that in the early centuries or millenniums of the 
Huronian there arose from some aggregated or compound 
infusorian, the prototype of the sponges. 
From some primitive gastrula, which became fixed to the 
Huronian. sea bottom, may have arisen the hydroid ancestor of 
the Ceelenterates ; owing to its fixed mode of life, the primitive 
digestive cavity opened upwards, being held in place by the 
septa, so that the vase-shaped body, growing like a plant, with 
the light striking upon it from all sides, assumed a radial 
symmetry. Before the beginning of the Cambrian, for we 
know Aurelia-like forms abounded on the Cambrian coasts, 
medusz budded out from some hydroid polyps, became free 
swimming, and as a result of their living at the surface became 
transparent, and thus shielded from the observation of whatever 
enemies they had, multiplied in great numbers. 
From some reptant gastræa there may have sprung, in these 
primeval times, an initial form with a fore-and-aft, dorso-ventral 
and bilateral symmetry, which gave origin by divergent lines of 
specialization to flatworms, nemerteans, and roundworms, âs 
well as Rotifera, and other forms included among the Vermes. 
It is probable that the trematodes and cestodes, especially the 
1 The Fauna of the Lower Cambrian or Olenellus Zone. Tenth Ann. Rep. U.S. 
Geological Survey, 1888, 1889. 
