August 26, 1898.] 



SCIENCE. 



253 



foreshadowed by submarine plateaus cov- 

 ered by shallow seas, the deeper portions of 

 the ocean basins not being affected by these 

 oscillations, extensive as they were. 



The time which elapsed between the end 

 of the Laurentian and the beginning of the 

 Cambrian was immense, or at least as long 

 as the entire Paleozoic era. Walcott esti- 

 mates 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 zoologist 

 may be allowed for the moment free scope 

 to act. It is perhaps not hazardous to sur- 

 mise that ia the early centuries or millen- 

 niums of the Huronian there arose, from 

 some aggregated or compound infusorian, 

 the prototype of the sponges. 



From some primitive gastrula which be- 

 came fixed to the Huronian sea-bottom may 

 have arisen the hj'droid ancestor of the 

 Coelenterates ; 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 radical symmetry. Be- 

 fore the beginning of the Cambrian, for we 

 know Aurelia-like forms abounded on the 

 Cambrian coasts, medus£e budded out from 

 some hydroid polyps, became free-swim- 



* The fauna of the Lower Cambrian or Olenellus 

 zone. Tenth Annual Report of the U. S. Geological 

 Survey, 1888-89. 



ming, 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 num- 

 bers. 



From some reptant gastrsea there may 

 have sprung, in these primeval times, an 

 initial form with a fore-and-aft, dorso-ven- 

 tral and bilateral symmetry, which gave 

 origin by divergent lines of specialization 

 to flat-worms, nemertean and round-worms, 

 as well as Eotifera, and other forms included 

 among the Vermes. It is probable that the 

 Trematodes and Cestodes, especially the lat- 

 ter, whose organs have undergone such re- 

 duction by parasitism, and some of which 

 through disuse have totally disappeared, 

 did not evolve until some time after the 

 appearance of molluscs and fishes. 



When existence in these early plastic 

 vermian forms was confined to boring in the 

 mud and silt, the body became cylindrical, 

 as in some nemerteans, and in the thread- 

 worms ; some of the latter forms, boring into 

 the mud, became parasites, entering the 

 bodies of other animals which serve as their 

 hosts. 



At about this time certain worms, as the 

 simple mechanical result perhaps of thread- 

 ing their way over or through the rough 

 gravelly bottom, became segmented. The 

 establishment of a segmental structure, 

 brought about by the serpentine mode of 

 progression in the direction of least resist- 

 ance, resulted in the origination of a suc- 

 cession of levers. Following this annulated 

 division of the dermo-muscular tube of 

 worms, was the serial or segmental arrange- 

 ment of the internal organs, i. e., the nerv- 

 ous, excretory, reproductive and glandular, 

 and, in a less degree, the circulatory. 



In certain of these primitive protanne- 

 lids, as the result perhaps of external 

 stimuli intermittently applied, bristles orig- 

 inated to aid in progression, and finally the 

 segmentally arranged lateral flaps of the 



