128 ESSAY ON CLASSIFICATION. 



condition of relative inferiority, when contrasted with 

 what it grows to be after it has completed its develop- 

 ment, 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 with- 

 out cirrhi, supporting a globular head, upon which the 

 so-called arms are next developed and gradually com- 

 pleted by the appearance of branches ; a few cirrhi are at 

 the same time developed upon the stem, winch increase in 

 number until they form a wreath between the arms and 

 the stem. At last, the crown having assumed all the 

 characters of a diminutive Comatula, drops off, freeing 

 itself from the stem, and the Comatula moves freely as an 

 independent animal. l 



The classes of Crustacea and of Insects 2 are particularly 

 instructive in this respect. Eathke, however, has described 

 the transformations of so many Crustacea, that I cannot 

 do better than refer to his various papers upon this sub- 

 ject, 3 for details relating to the changes these animals 

 undergo during then' 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, the Entomostraca and Isopoda, 

 it next assumes the shape of those of a higher order, the 



1 A condensed account of the trans- . 3 See AGASSIZ'S Twelve Lectures, 



formations of the European Coma- p. 62, and Classification of Insects, 



tula maybe found in E. FORBES'S etc., q. a. It is expected that Embry- 



History of the British Starfishes, p. ology will furnish the means of as- 



10. The embryology of our species certaining the relative standing of 



will be illustrated in one of the next every family, 



volumes of my contributions to the 3 See above, page 119, note 2. 

 Natural History of the United States. 



