II 



METAZOA 



87 



The simplest members of the Metazoa, in which most of the pecul- 

 iarities mentioned above as characteristic of the typical Metazoa have 

 not yet made their appearance, constitute the phylum Coelenterata. 

 Amongst these in turn some of the very simplest are the Hydrida 

 exemplified by the genus Hydra. 



Hydra 



Hydra is the common little " fresh-water polyp " which inhabits 

 ditches, ponds, lakes, 

 all over the world. 

 When in its normal 

 condition (Fig. 32, A) 

 it is tubular in form — 

 one, closed, end of the 

 tube adherent to some 

 solid object such as a 

 water-plant or stone, 

 the other with a minute 

 pore — ^the mouth (M) 

 — situated at the apex 

 of an oral cone. The 

 mouth is closed and 

 practically invisible 

 except when it is 

 distended during the 

 taking in or rejection of 

 food material. Round 

 the base of the oral 

 cone are attached 

 about six to eight deli- 

 cate threadlike (really 

 tubular) tentacles (Fig. 

 32, A, 0- To study the 

 structure of the Hydra 

 in detail it is necessary 

 to treat specimens with 

 some substance such 

 as Acetic acid which 

 will loosen the cohesion 

 of the cells and allow them to be broken apart by tapping on the 



Fig. 32. 



HyUra. X 12. A, Partially extended ; B, contracted. (From 

 Graham Kerr's Primer of Zoology.) c, Coelenteron ; ect, ectoderm ; 

 end, endoderm ; JW, mouth ; /, tentacle. 



