CNIDARIA 



87 



Fig. 146 The action of nettle 

 organs (Toppe). A, a nettle or- 

 gan piercing the chitinous shell 

 of an insect : B, nettle organs 

 holding a small crustacean by its 

 spines. 



sufficient force to puncture the skin of small animals (Fig. 146)", and the 

 poisonous fluid which is thus injected into the wound may cause paralysis 

 or death. The thread tubes also often 

 lasso small animals by winding around 

 hairs and other projections of their 

 bodies (Fig. 146, B). The nettle organs 

 of the larger jellyfish often inflict a 

 painful wound to man. 



The body of the Cnidaria is dis- 

 tinctly radiate in structure; in the 

 smallest jellyfish and hydroid polyps the 

 number of radii is usually four and in 

 the larger ones and the Alcyonaria, some 

 multiple of four; in the Zoantharia it is 

 usually six or a multiple of six. 



Two distinctly different types of 

 structures are present among the Cni- 

 daria, which are, however, capable of 

 being referred to a common fundamental 

 form. These are: (1) the hydroid or 

 polyp type (Fig. 147, A), and (2) the medusa or jellyfish type (Fig. 147, 

 B, or Fig. 154). The first is seen in the simplest form in the fresh- water 

 Hydra and the hydropolyps, and in a more complex form in the corals ; in 



these animals the body is 

 cylindrical in form, one end 

 is usually attached to some 

 more or less stationary ob- 

 ject, while at the other end 

 is the mouth surrounded by 

 tentacles. 



Two variations of this 

 type are found. In the 

 Scyphozoa and the An- 

 ihozoa (Fig. 219) longitu- 

 dinal mesenterial ridges pro- 

 ject prominently into the 

 gastrovascular space and a 

 gullet lined with ectoderm 

 is present, while in the sim- 

 pler Hydrozoa or Hydro- 

 medusae these features are wanting (Fig. 147, A). Cnidarians of the 

 hydroid type live in colonies in the majority of cases which often con- 



Fig. 147 The two types of structure of cni- 

 darians. A, the hydroid type ; B, the medusa 

 type, a quarter of the animal being removed to 

 show the internal structure. (Delage et H6rou- 

 ard.) 1, mouth; 2, gastrovascular space; 3, ten- 

 tacle; 4, exumbrella ; 5, subumbrella ; G, manu- 

 brium ; 7, velum. 



