XIV INTRODUCTION. 
animal nature of the sponges there are sufficient 
grounds for considering them animals and as 
belonging to this class, and they are omitted 
here only from a desire of further investigation- 
The British polypous Zoophytes naturally 
divide themselves into two grand divisions oi' 
sub-classes; the first embraces the radiated 
form, in which the body is contractile in every 
part with but a single aperture. This division 
contains three minor ones, or orders, the Ify' 
droicla, Asleroida, and Helmut hoida. The se- 
cond division contains the Molluscan Zoophyte A 
in which the body is non-contractile, non-syD*' 
metrical, with two apertures. It has only on e 
order, the Ascidioida which is very closely 
allied to the Mollusca lunicaia. In the Hy droid j* 
the tentacula are tuberculated, and the stomach 
a mere depression made in the granular m9 sS 
without any particular organization, and th e 
horny sheath which is not found in all the genera 
is external. In the Asleroida, the tentacula al ' e 
eight in number and fringed ; the solid pal' 1 ® 
when present, are all internal; and the extern 3 * 
surface is always marked with eight rayed d e ' 
pressions. The Helianthoid having for 
type the common Anemone, need hardly be i' € ' 
fered to now. In the second sub-class there 
a considerable advance in the complication 0 
the polype. In the orders of the first division* 
the tentacula are prehensile and used only 
capturing prey; in those of the second, they 3l ’ e 
ciolhed with vibratile cilia and are chiefly sub' 
servient to respiration. In the first three order-’ 
the polypes are comparatively sluggish, in 
last they are remarkable for the rapidity of the* 1 
actions. . 
Though these productions are low in the sc 3 * t 
of being, and the polypes for the most p 3 ’ 
