314 CCJELENTERATA chap. 



what is called an alternation of generations, the asexual genera- 

 tion or agamohiinn (hydroid colony) giving rise by budding 

 to the sexual generation or gamobiiim (medusa), which in its 

 turn produces the agamobium by a sexual process, i.e. by 

 the conjugation of ovum and sperm. 



Hydra and Bougainvillea 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 mesogloea, 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 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 Coelomata. 



PRACTICAL DIRECTIONS. 

 Hydra. 



Examine some living HydrK in a vessel of w ater, with the naked e3e 

 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. 



