4 68 



CHARACTERS OF INVERTEBRATE ANIMALS 



a space corresponding to the digestive cavity or gut, which, 

 however, is not bounded directly by the wall of the body but by 

 a wall of its own separated by a considerable space, the body-cavity, 

 from the body- wall. Hydra is absolutely devoid of a body-cavity 

 in this sense. If we attempt a comparison between the two 



animals, the epidermis of the 

 worm may be looked upon 

 as ectoderm, and the inner- 

 most layer of the wall of the 

 gut would appear to be equi- 

 valent to endoderm. But the 

 greater part of body -wall 

 and gut -wall in the worm, 

 together making up what is 

 called the middle layer or 

 mesoderm, has no distinct 

 representative in Hydra, 

 unless, perhaps, it is repre- 

 sented in an inadequate 

 manner by the thin mem- 

 brane between ectoderm and 

 endoderm. The presence of 

 a distinct third layer coming 

 between ectoderm and endo- 

 derm is characteristic of all 

 the phyla above the Zoo- 

 phytes, and this comes out 

 very clearly and simply in 

 embryo stages. Hence all 

 these phyla are collectively 

 grouped together as Trip lob las tica, i.e. three-layered animals, as 

 contrasted with the two-layered forms or Diploblastica. The 

 body-cavity is simply a split in the mesoderm, dividing it into an 

 outer sheet lining the ectoderm and an inner sheet investing the 

 endoderm. 



Allusion has already been made to the fact that the body of 

 a higher animal is made up of a number of different sorts of 

 material having specific purposes and known as tissues, which are 

 in effect the building materials from which the various organs are 

 constructed. They are as follows: i. Epithelium, consisting of 



Fig. 287. Diagrams, enlarged to various scales, illustrating 

 the structure of Hydra 



A, Longitudinal section: ectoderm left white, endoderm re- 

 presented in black. B, Small part of longitudinal section through 

 body- wall (the black dots are nuclei); ECT., three large cells and 

 two packing cells are seen ; SUP. LAYER, supporting layer or 

 lamella; END., endoderm, showing four cells with flagella, and 

 one with pseudopodia. c, A nettling-cell, with thread pro- 

 truded note barbs at the base of this, and the trigger hair on 

 the right side ; the protoplasm investing cell is shaded, and the 

 nucleus represented in black. 



