322 CCELENTERATA CHAP. 



Hydra and Obelia both belong to the simplest class 

 the Hydrozoa of the phylum Coelenterata : this phylum 

 includes all the polypes or zoophytes, the jelly-fishes, 

 and the anemones and corals. In all there is an 

 ectoderm and an endoderm, separated by a mesoglcea, 

 which may consist, as in Hydra, of a structureless 

 membrane containing no cells, or may be gelatinous 

 as in the medusa, and may even contain cells, thus 

 assuming more the character of an intermediate cell- 

 layer or mesoderm. There is no body-cavity or ccelome 

 (p. 20) surrounding the digestive cavity or enteron, 

 and tentacles are present round the mouth. Organs of 

 offence occur in the form of thread-cells or nematocysts. 



In all the higher phyla a definite mesoderm is 

 developed in the embryo in addition to the ectoderm and 

 endoderm (triploblastic condition), and in nearly all cases 

 there is a definite cavity or coelome present in the 

 mesoderm : hence all these animals are often included 

 together as the Coelomata. 



PRACTICAL DIRECTIONS 

 Hydra. 



Examine some living Hydrae in a vessel of water, 

 with the naked eye or with a pocket lens, and note the 

 differences in form according to the degree of contraction. 

 The animal is usually attached to foreign bodies (weeds, 

 &c.) at one end, and at the other end a number of tentacles 

 (usually six to eight) are given off. In the expanded state 

 the body and tentacles are greatly elongated and thread- 

 like, while when contracted the body is more globular, and 

 the tentacles appear like small knobs. Note the brown 

 colour in H. fusca, and the green colour in H. viridis. 

 Observe the method of seizing food. Place a specimen on 

 a. slide in a drop of water, together with a small piece of 

 water-weed or paper to prevent crushing, and then put 

 on a cover-glass. Wait till the animal is fully expanded, 

 and then examine with the low power. Note (Fig. 75) : 



i. The body, enclosing the digestive cavity or enteron, which 

 opens by the mouth at the distal end of the animal, at the 



