52 INVEETEBRATE ANIMALS. 



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. 

 13, a). In other cases there intervenes between the mouth 

 and the body-cavity a short alimentary tube, which communi- 

 cates externally with the outer world through the mouth, and 

 opens below by a wide aperture into the general cavity of the 

 body. In no case is there a distinct intestinal tube which 

 runs through the body-cavity and opens on the surface by a 

 mouth at one end and an excretory aperture or anus at the 

 other. Another leading character of the Ccelenterata is the 

 composition of the body out of two fundamental membranes 

 (Fig. 13), 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 microscopic organs called " thread-cells," or " net- 

 tle-cells," which communicate to many of the Ccelenterata 

 (such as the sea-jellies) their peculiar power of stinging, are 

 structures found in the integument of almost all the mem- 

 bers of this silb-kingdom, and sometimes in internal parts as 

 well. They are very beautiful objects of microscopical ex- 

 amination, 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. 13, d) are " oval elastic sacs, con- 

 taining 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 sufBcient 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 Ooelenterata are 

 divided into two great classes, termed respectively the Sy- 

 drozoa and the Actinozoa. In the Hydrozoa, there is no 

 body-cavity distinct from the digestive cavity — or, in other 

 words, the body-cavity is the digestive cavity. In the Acti- 



