70 POPULAR ILLUSTRATIONS OF 



Hitherto we have found the food carried either into the general 

 cavity of the body, or into vacuoles interspersed through its sub- 

 stance. In the .Coelenterata, however, the body substance consists 

 of two layers one an outside or integumentary organ, or skin, 

 called the ectoderm ; the other lining a cavity which is always pre- 

 sent, and which, communicating with the mouth by means of a 

 gullet, constitutes the stomach. This inner layer of body substance 

 is called the endoderm. Now these two layers consist of vesicular 

 bodies embodied in a matrix, the inner one having the cells applied 

 to each other more closely, and increasing from without inwards, 

 while in the outer layer they are more loosely scattered through the 

 body substance or matrix, and increase from within outwards. If 

 the reader will bear these simple facts in mind he will have a know- 

 ledge of the great difference which exists between this sub-kingdom 

 and the last. By these two layers most complicated organs are 

 developed. They produce deadly thread or poison cells, which have 

 the power of benumbing their prey ; they produce reproductive 

 organs, colouring or pigment bodies, and granular matters, which 

 are intimately connected with the great function of secretion ; they 

 can also protect themselves outwardly by organs of great beauty, or 

 develop a skeleton to support them, as in the great group of cal- 

 careous, curiously and elegantly built-up bodies known as Corals. It 

 is, in fact, upon the history in space and time of these structures 

 that I shall principally dwell in illustrating the Coelenterata. But 

 I will take a general view of the whole family, as its members 

 are rather celebrated for qualities which excite our interest and 

 curiosity. 



The Coelenterata are divisible by anatomical differences as to the 

 connection of the two layers with each other into two classes viz., 

 (1) the Hydrozoa and (2) the Actinozoa. 



1. THE HYDEOZOA. 



The type of this class may be found in any stagnant ditch. 

 It is known by the name of Hydra, or fresh-water polyp. It 

 seldom exceeds, without its long tentacles, three-quarters of an 

 inch in length. Its body is gelatinous, but endowed with great 

 powers of contractility, and its stomach is capacious. From the 

 ectoderm, or outer skin, it has the power of projecting vast num- 

 bers of thread-like organs, with which it benumbs or kills its 

 prey. These are especially placed along the outer covering of the 

 long tentacles by which the prey is caught (Figs. 88-92), and 

 are projected by a beautiful mechanism presently to be described. 

 These tentacles spring from a margin immediately below the open- 

 ing which serves as a mouth (a, Fig. 88), and are spread out as 



