RADIATA. 25 
Trachelocerca 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. 
Paramecium 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 Lotifer vulgaris (pl. 75, fig. 16), 
remarkable for the two circles of vibrille already referred to, and for the 
posterior forceps by which it attaches itself. Melicerta ringens ( fig. 19) 
ean withdraw itself into an external case; it lives in society, and has the 
vibrillee distributed in four divisions. 
Drviston I. Raprata. 
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 Medusze, 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 
Worms, 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, agreeing 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 differ in 
the manner in which the plan is carried out.”—Agassiz’s Lectures on 
Embryology, Boston, 1849. P. 43. 
The Radiata are distributed into three classes, Colenterata, Zoophyta, 
and Echinodermata. The first includes the Acalephe or Meduse, to which 
the Hydroida are added ; the second the Zoophyta, excluding the Hydroida; 
and the third the Echinodermata. 
The Hydroida 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 Acalephe, which 
in their turn have been regarded as an individual class. In dismembering 
the Zoophyta to unite the Hydroidaand Acalephe, 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 Naked-eyed Meduse, 
p- 82; and in Agassiz’s Lectures on Embryology, p. 44. 
229 
