Chap. I. METAMORPHOSES OF ANIMALS. 85 



these changes, and has furnished a standard to determine their relative rank ; as 

 it cannot be doubted, that the earher stages of growth of an animal exhibit a 

 condition of relative inferiority, when contrasted with what it grows to be, after 

 it has completed its development, and before it enters upon those phases of its 

 existence which constitute old age, and certain curious retrograde metamorphoses 

 observed among parasites. 



In the young Comatula there exists a stem, by which the little animal is 

 attached, either to sea weeds, or to the cirrhi of the parent; the stem is at first 

 simple and without cirrhi, supporting a globular head, upon which the so-called arms 

 are next developed and gradually completed by the appearance of branches; a few 

 cirrhi are, at the same time, developed upon the stem, which increase in number 

 until tlie}^ form a wreath between the arms and the stem. At last, the crown 

 having assumed all the charactex-s of a diminutive Comatiila, drops off, freemg itself 

 from the stem, and the Comatula moves freely as an independent animal.^ 



The classes of Crustacea and of Insects,^ are particularly instructive in this 

 respect. Rathke, however, has described the transformations of so many Crustacea, 

 that I cannot do better than to refer to his various papers upon this subject,^ for 

 details relating to the changes these animals imdergo during their earlier stages of 

 growth. I would only add, that while the embryo of the highest Crustacea, the 

 Brachyura, resembles by its form and structure the lowest types of this class, as the 

 Entomostraca and Isopoda, it next assumes the shape of those of a higher order, 

 the Macroura, before it appears with all the characteristics of the Brachyura. 



Embryology furnishes, also, the best measure of the true affinities existing 

 between animals. I do not mean to say, that the affinities of animals can only be 

 ascertained Ijy embryonic investigations ; the history of Zoology show.s, on the con- 

 trary, that even before the study of the formation and growth of animals had 

 become a distinct branch of physiology, the general relationship of most animals had 

 already been detennined, with a remarkable degree of accuracy, by anatomical inves- 

 tigations. It is, nevertheless, true, that in some remai'kable instances, the knowledge 

 of the embryonic changes of certain animals gave the first clue to their true affini- 

 ties, while, in other case.s, it has furnished a very welcome confinnation of relation- 

 ships, which, before, could appear probable, but were stiU very problematical. Even 

 Cuvier considered, for instance, the Barnacles as a distinct class, which he placed 



' A condensed account of the transformations of ^ See Agassiz's Twelve Lectures, p. C2, and 



the European Comatula, may be found in E. Classification of Insects, etc.. (j. a. It is expected 



FoRBEs's History of the British Starfishes, p. 10. tliat Enibrj-ology may furnish the means of ascer- 



The embryology of our species will be illustrated tnining the relative standing of every family, 

 in one of my next volumes. ' See above, page 79, note 2. 



