52 



INVEKTEKIIATE ANIMALS. 



there a distinct intestinal tnl.ie wliicli inns tln-ongli the body-ciivity 

 and opens on the surface by a nioutli at one end and an excretory 

 aperture or anus at tlie other. It may be added in this connection 

 that, though it is conveiiient to api)ly tlie term of " body-cavity " to 

 the general space included within the bi)dy-walls of a ( 'celenterate 

 animal, the space dues not cori'espond in its mode of develojiment 

 with what is known as the liody-caxity of the higher animals. 



Another leading character fjf the (Jiflmitfriita is the composition 

 of the body out of two fundamental membranes (fig. 26), which are 

 usually of a very simple structure in the emliryo, liut which in the 

 adult become moi-e or less comi)licated by the development of mus- 

 cular fibres and other tissues. The outer of these laj'ers or mem- 

 branes is known as the " ectoderm," and it forms the whole of the 



Fig. 27.— Tlnvail-cc-ll.s of Cd-lci 

 CL-11 of a i-oral ((.V(;//o^//i,v//m 

 Spa-aneiiKme (C'tTiillirnnrjiln 

 cell otHndra, with the tliiv 



otiter surface of the lifidy 

 The inner l.-ij'er is ktiowi 

 of the interior of the bi 

 tacles round the mouth, 

 the enihidcrni, are ustiidl 

 ing cilia. The muscular 

 twi'cn the cctodo'm am 

 " nicsiideriu." The jiecul 

 " ni'l lle-ci'lls," or " cnida', 

 tvra/ii I'suc'li as tin 



itfiate animals, ^-rpatly iiiagiiifn-.l. A and B, Thread. 

 0, witli tlie tlireail everti'.l C and D, Tlireail-eell .if a 

 rs), in ils iniieseent and active eonditiL'U. E, Thread- 

 ad ev.'l'led." 



, terminating at the margins of the mouth. 

 I as the " emldderni," and it lines the whole 

 idy, lieiug ])rolonged into the tubular ten- 

 Biitli of these menibi'anes, but especially 

 y mcirc nr Ic-is richly furni.shed with vibrat- 

 and other tissues which ai-e developed be- 

 1 endoderm, coiistitute what is c died the 

 iar luiciTiscnpic (irgans called "thread-cells," 

 " which connnunic.ate to nianv of the Ctilen- 



j'llii'si tlirir ]icculiar ]iower of stinging, are 



