^514 QELENTKRATA rn.w. 



wliiit is called an alleniatioii of generaliiDis, the asexual genera- 

 tion or aganiobliiin (h\(lroid colon\) cj;i\ing rise by budding 

 to the sexual generation or gaino/>iuni (medusa), which in its 

 turn produces the agamobium by a se\ua1 process, i.e. by 

 the conjugation of ovum and sperm. 



Hydra and Bougainvillea both belong to the simplest 

 class — the Hydrozoa—oi 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 mesogkea, which may 

 consist, as in H)dra, of a structureless membrane containing 

 no (ells, or ma\' be gelatinous as in the medusa, and may 

 even contain lells, thus assuming more the character of an 

 intermediate cell-la}'er or mesoderm. There is no body- 

 ca\ity or ca'/oinc (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 ph)la a definite mesoderm is developed 

 in the embryo in addition to the ectoderm and endoderm 

 {triplolilastic condition), and in nearly all eases there is a 

 definite cavitv or cielome present in the mesoderm: hence all 

 these animals are often included together as the Coelomata. 



I'KAriKWL DIKKC'IIOX.S. 



Hydra. 



[■AaniiiiL- SOUK* li\"ini; Ihilr^v.' in a \ l\s,sl^1 nf water, \\itli the naked e}e 

 ■ ir willi .1 jjnrket lens, and iir.te the dirferenees in tovni aceording ti^ 

 the degree f>f eontractjiui. The aiiirnal is nsuall\ attached to foreign 

 liodies (weeds, c\:c. ), at one end, and at the olheicnd a niniil)er of tenta- 

 cles (usually six lo eight) are given off. In the expanded .state the body and 

 lenlarles are greatly elciiigaled and lliread-like, « hile when contracted 

 the Ijudy is mure gloladar, and the tentacles appear like small knobs. 



