130 AN INTRODUCTION TO ZOOLOGY. 



II. Type of the Ccelenterata : second grade of ani- 

 mal life, diploblastic animals with no distinction 

 between body-cavity and alimentary canal. 



Ccelenterata : animals more or less resembling tlie Fresh- 

 water Polyp, Hydra (various species, uM'icZi's, /itsca, etc.). See 

 fig. 11, p. 46, fig. 30, p. 149. 



Hydraia a small cylindrical animal with a mouth, and arms 

 called tentacles. It agrees with all other types of Metazoa, in 

 being a multicellular organism, and in having special cells, 

 the ova and spermatozoa, differentiated for the purpose of 

 sexual repi'oduction ; but it differs from them, in the follow- 

 ing particulars : viz., it is diploblastic (p. 29) ; it presents no 

 distinction between body-cavity and food-canal (p. 40), the 

 digestive cavity being a simple sac ' with no opening besides 

 the mouth (p. 168) ; the ectoderm, especially of the tentacles, 

 is armed with stinging structures called nematooysts^ (p.l48), 

 and there is a perfect radial symmetry," shown in the pre- 

 sence round the mouth of a ring of tentacles, usually six or 

 eight in number, each of which is lined by endoderm, and 

 contains a hollow space prolonged from the enteric cavity. 



Although Hydra is described as diploblastic, the third body- 

 layer of higher animals is to some extent represented by a 

 thin " supporting lamella " or mesogloea (p. 143) ; this lies 

 next to a layer composed of contractile processes sent out by 

 the ectoderm cells, which constitute a rudimentary form of 

 muscle. Certain cells in the ectoderm are interpreted as 

 nerve-cells ; while others (cnidoblasts) give rise to the nemato- 

 cysts already named. Thus, although the body of Hydra 

 has not attained to the possession of " organs " (unless we 

 may dignify with that name the groups of cells which form 

 the ovaries and spermaries), yet the cells of the ectoderm 

 have attained a considerable degree of differentiation. The 

 cells of the endoderm. which are remarkable for presenting 



I - ^ Hence the uames CcEleutcrata, Ciiiilaria, Eadiata. 



