248 



GENERAL BIOLOGY 



to seven. These are closed at their free ends, but their interior channels 

 are a direct continuation of the gastro-vascular cavity. 



At the distal third of the body, the male gonads, the testes, are seen 

 as cone-shaped elevations during the breeding season (September and 

 October), while the female gonads, the ovaries, are knoblike projections 



close to the basal disk. In addition 

 to these sexual organs one may find 

 buds on various parts of the body. 



As the Hydra is a diploblastic 

 animal, that is, one which has re- 

 mained in the gastrula stage, it 

 means that the simple indentation 

 of the original blastula has given the 

 animal only epithelial tissue, for 

 epithelium is surface tissue, and 

 both inner and outer portions of this 

 animal are surface tissues. 



The ectoderm is primarily pro- 

 tective and sensory, and is made up 

 of two principal kinds of cells: (1) 

 epitheliomuscular, and (2) intersti- 

 tial. The former are shaped like in- 

 verted cones, and possess long (up 

 to .38 mm.), unstriped contractile 

 fibrils at their inner ends ; these 

 enable the animal to expand and 

 contract. 



The interstitial cells lie among 

 the bases of the epitheliomuscular 

 cells ; they give rise to three kinds 

 of nematocysts or stinging cells 

 (Fig. 153). Nematocysts are present on all parts of the body except the 

 basal disc, being most numerous on the tentacles. The interstitial cell 

 in which the nematocyst develops is called a cnidoblast ( ) ; 



it contains a nucleus and develops a trigger-like process, the cnidocil 

 ( ), at its outer end, but is almost completely filled 



by the pear-shaped nematocyst. Within this structure is an inverted 

 coiled thread-like tube with barbs at the base. When the nematocyst 

 explodes, this tube turns rapidly inside out and is able to penetrate the 

 tissues of other animals. The explosion is probably due to internal pres- 

 sure produced by osmosis, and may be brought about by various methods 

 such as the application of a little acetic acid or methyl green. Many 

 animals when "shot" by nematocysts are immediately paralyzed and 

 sometimes killed by a poison called hypnotoxin which is injected into it 

 by the tube. 



Fig. 152. Hydra. 



A, an animal in its expanded form ; B, 

 the same animal contracted ; C, a diagram of 

 the longitudinal section of the animal, show- 

 ing the internal structure ; D, an epithelio- 

 muscle cell ; E, & bit of the body wall highly 

 magnified showing the two layers of the body ; 

 F, a digestive cell ; G, one of the nemato- 

 cysts with its thread extruded ; H , & second 

 type of nematocyst with the coiled thread 

 within the sac ; I, nematocyst of the third 

 type with its thread extruded ; J, a bit of the 

 tentacle, very highly magnified, showing the 

 batteries of the nematocysts ; K, two of the 

 secreting cells of the basal disk, en, cnidocil ; 

 ec, ectoderm ; en, endoderm ; m, mouth ; mes, 

 mesogloea ; o, ovary ; s, spermary ; t, new ten- 

 tacle forming. (After Conn.) 



