. The causes and course of organic evolution; . for reasons that cannot be enlarged on now, we acceptit that these, like the Spongida, became a side line of evolu-tion, and in persisting from the cambrian to the present periodhave slowly built up reefs and island masses, but have failedto evolve higher organisms than themselves. A somewhat similar history attaches to the Echinodermata,except that the earliest ancestral members seem to be entirelyunknown, and so our existing ones are all derived secondarilyfrom ancient marine organisms, but whether these were in 506 Causes and Course of Organic
. The causes and course of organic evolution; . for reasons that cannot be enlarged on now, we acceptit that these, like the Spongida, became a side line of evolu-tion, and in persisting from the cambrian to the present periodhave slowly built up reefs and island masses, but have failedto evolve higher organisms than themselves. A somewhat similar history attaches to the Echinodermata,except that the earliest ancestral members seem to be entirelyunknown, and so our existing ones are all derived secondarilyfrom ancient marine organisms, but whether these were in 506 Causes and Course of Organic Evolution turn early evolved from fresh-water types is a question onwhich no light has yet been shed. A point is now reached where the palseontological and thezoological records are equally defective. For granting theexistence of archaean organisms that formed from the singleegg cell a morula, or cellular embryonic mass, as in youngphases of the Spongida, Coelenterata, and Echinodermata,few organisms seem to exist now which connect such with. soc -mc Fig. 20—Diagram of Rotifer after Zelinka: a-p apical plate or area; 6p basaldo.; 80C. anterior ciliated ring; 8ac posterior do.; m mouth; a anus of alimentarycanal; db dorsal brain or cerebral ganglion; vh ventral do.; e eyes; nep nephri-dia; eg cement glands. those higher and more complex alliances that evidently rep-resent the main line, as well as many collateral or diverginglines of ascent, like the Rotifera, the Turbellaria, and thePolyzoa, not to name higher classes. But larval or developmental stages of the higher classesseem clearly to point the way backward toward the Rotiferaas a great basic group, from which have sprung most of thedominant classes now existing. As leading up to this it maybe said that zoologists are now largely agreed in regarding theso-called trochophore stage of many large phyla or branchesof invertebrate animals as representative temporarily of what Phylogeny of Animals 507 once must have been a wi
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