26 



INTRODUCTION TO ZOOLOGY 



From some of tlie "packing cells" of the skin there 

 develop certain highly specialised cells known as dart cells 

 or thread cells — also called nematocysts (nema, a thread) 

 — which work their way through to the outer surface of 

 an ordinary skin cell (Fig. 11, B, d), and there perform 



Fig. 11. — Hydra. 

 A, Longitudinal section of the whole hody ; m, mouth ; 6, bud ; sp, spennary ; o, egg 

 cell inside ovary. B, A few cells of the body-wall enlarged ; s, skin cells or 

 ectoderm ; d, dart cell ; di, digestive cells or endoderm. C, One ectoderm cell 

 separated to show muscle processes. 



their special function of enabling the Hydra to catch and 



paralyse its prey. 

 Dart Cells Each dart cell consists of a little oval bag. From 

 or Nemato- the outer side of the bag, the wall is extended into 

 cysts. jt;s cavity as a hollow finger-like process, ending in 



