Transcriber's note: Italic text has been marked with _underscores_. Please see the end of this book for further notes. When I first accepted the invitation of the editors of the International Scientific Series to supply a book upon Primitive Nervous Systems, I intended to have supplemented the description of my own work on the physiology of the _Medusæ_ and _Echinodermata_ with a tolerably full exposition of the results which have been obtained by other inquirers concerning the morphology and development of these animals. But it soon became apparent that it would be impossible, within the limits assigned to me, to do justice to the more important investigations upon these matters; a…
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