322 CCELENTERATA CHAP. 



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 con- 

 sist, 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 nema- 

 tocysts. 



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 ccelome present in the mesoderm : hence 

 all these animals are often included together as the 

 Ccelomata. 



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 tenta- 

 cles (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 



