52 



INVEKTEBEATE ANIMALS. 



. thei'e a distinct intestinal tube which runs through the body-cavity 



i and opens on the surface by a mouth at one end and an excretory 



' aperture or anus at the other. It may be added in this connection 



that, though it is convenient to apply the term of " body-cavity " to 



the general space included within the body-walls of a Ctelenterate 



j animal, the space does not correspond in its mode of development 



with what is known as the body-cavity of the higher animals. 



Another leading character of the Calmtirnda is the composition 

 of the body out of two fundamental membranes (fig. 26), which are 

 visually of a very simple structure in the embryo, but which in the 

 adult become more or less complicated by the development of mus- 

 cular fibres and other tissues. The outer of these layers or mem- 

 branes is known as the '' ectoderm," and it forms the whole of the 



Fig. 2". — Thread-cells of Cueleiiterate animals, gi'eatly magnified. A and B, Thread- 

 cell of a I'oral (Caryophyllia), with the thread everted. C and D, Thread-CL'll of a 

 Sea-aneirione iCorallimorplius), in its quiescent and active condition. E, Thread- 

 cell of Hydra, with the thread everted. 



outer surface of the body, terminating at the margins of the mouth. 

 The inner l.iyer is known as the " endoderm,'' and it lines the whole 

 of the interior of the body, being prolonged into the tubular ten- 

 tacles round the mouth. Both of these membranes, but especially 

 the endoderm, are usually more or less richly furnished with vibrat- 

 ing cilia. The muscular and other tissues which are developed be- 

 tween the ectoderm and endoderm, constitute what is called the 

 " mesoderm." The peculiar microscopic organs called " thread-cells," 

 " nettle-cells," or " cnidt^," which communicate to many of the Ccelen- 

 ti-i-iii'i. (such as the sea-jellies) their peculiar power of stinging, are 



