HADIATA. 25 



TracJieloGerca viridis {fig. 6) is more rare than the preceding, and 

 inhabits the same places. It takes its trivial name from the green germs 

 within it. 



Parmnecium compressum {fig. 9), already alluded to, is from one twentieth 

 to one twenty-fourth of a line long, and is probably an early stage of Planaria. 



The structure of the two next species figured among the Infusoria shows 

 that they are Crustacea. The first is Rotifer vulgaris {])l. ^i^.,fig. 16), 

 remarkable for the two circles of vibrillse already referred to, and for the 

 posterior forceps by which it attaches itself. Melicerta ringens {fig. 19) 

 can withdraw itself into an external case ; it lives in society, and has the 

 vibrillse distributed in four divisions. 



Division I. Radiata. 



The radiated division of the animal kingdom, in the arrangement of 

 Agassiz, and to a certain extent, in that of Milne Edwards, includes all 

 those forms in which the radiated structure is more or less evident, as in 

 the Zoophyta, the Medusa:", and the Echinodermata. In Cuvier's arrange- 

 ment, the Zoophyta (under which term he includes all the Radiata) are a 

 heterogeneous assemblage of radiated forms, Epizoic Crustacea, Intestinal 

 AVorms, and Infusoria. 



" In a general point of view, we may, however, compare further, all 

 radiated animals, when we shall find that they really constitute a natural, 

 well circumscribed group in the animal kingdom, agreehig in all important 

 points of their structure, being strictly constructed upon the same plan, 

 although the three classes which we refer to this great department dift'er in 

 the manner in which the plan is carried out." — Agassis^s Lectures on 

 Emlryology, Boston, 1819. P. 43. 



The Radiata are distributed into three classes, Colenterata.. Zoophyta.^ 

 and Echinodermata. The first includes the Acalephse or Medusse, to which 

 the Hydroida are added ; the second the Zoophyta, excluding the Ilydroida ; 

 and the third the Echinodermata. 



The Ilydroida have been usually placed in the class Zoophyta, although 

 in the development of some of the families in which it has been observed, 

 they present characters indicating a great affinity with the Acalej)h[c, which 

 in their turn have been regarded as an individual class. In dismembering 

 the Zooi^hyta to unite the Ilydroida and Acalephse, we may either apply the 

 name of the latter to the united group, or choose a distinct one. The latter 

 course is preferable, being least likely to cause confusion, and we have 

 accordingly adopted the name proposed by Fry and Leuckart. The necessity 

 of this union is insisted upon by Forbes in his British jS^aked-eyed Medusae, 

 p. 82 ; and in Agassiz's Lectures on Embryology, p. 44. 



229 



