56 INVERTEBRATE ANIMALS. 
the mouth opens into, and thus communicates freely with, the 
body-cavity. In some cases the mouth opens straight into the 
general body-cavity, which then serves as a digestive cavity as 
well (fig. 16, a). In other cases there intervenes between the 
mouth and the body-cavity a short alimentary tube, which com- 
municates externally with the outer world through the mouth, 
and opens below by a wide aperture into the general cavity of 
the body (fig. 15). In no case is there a distinct intestinal 
tube which runs through the body-cavity and opens on the sur- 
face by a mouth at one end and an excretory aperture or anus 
at the other. Another leading character of the Celenterata is 
the composition of the body out of two fundamental membranes 
(fig. 15), which are usually of a very simple structure, but 
which may be more or less complicated by the development of 
muscular fibres and other tissues. The outer of these layers or 
membranes is known as the “ectoderm,” and it forms the whole 
of the outer surface of the body, terminating at the margins of 
the mouth. The inner layer is known as the “ endoderm,” and 
it lines the whole of the interior of the body, being prolonged 
into the tubular tentacles round the mouth. Both of these 
membranes, but especially the endoderm, are usually more or 
less richly furnished with vibrating cilia. The peculiar micro- 
scopic organs called “ thread-cells,” “ nettle-cells,” or “ cnidae,” 
which communicate to many of the Celenterata (such as the 
sea-jellies) their peculiar power of stinging, are structures found 
in the integument of almost all the members of this sub- 
kingdom, and sometimes in internal parts as well. They are 
very beautiful objects of microscopical examination, and differ 
very considerably in the details of their structure. They are, 
however, in most respects essentially the same as in the common 
Hydra or fresh-water polype, in which the thread-cells (fig. 16, 
a) are “oval elastic sacs, containing a long coiled filament, 
barbed at its base and serrated along its edges. When fully 
developed the sacs are tensely filled with fluid, and the slightest 
touch is sufficient to cause the retroversion of the filament, 
which then projects beyond the sac for a distance, which is not 
uncommonly equal to many times the length of the latter” 
(Huxley). 
In accordance with the above-mentioned differences in the 
arrangement of the digestive system, the Cwlenterata are divided 
into two great classes, termed respectively the Hydrozoa and 
