22 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Within some of the interstitial cells (cnidoblasts) oval vesicles 

 (nematocysts) are developed, each of which contains a coiled-up 

 filament. Nematocysts are chiefly found in the distal end of the 

 body, especially in the tentacles, where they are aggregated into 

 " batteries," giving rise to an irregular surface. If a little weak 

 acid is added to the water which contains a Hydra, the threads 

 will be rapidly shot out. 



The cnidoblasts are at first situated deeply in the ectoderm, 

 but as their nematocysts (" thread-cells ") develop, force their way 

 to the surface. Several kinds of nematocyst may be distinguished, 

 of which the largest consist of a transparent vesicle containing 

 fluid, and (when examined in the discharged condition) this it 

 seen to be produced (D) into a long tapering neck, which towards 

 its end bears a few large backwardly directed spines, and is 

 continued into an extremely long and delicate hollow filament 

 or thread. In an unused nematocyst, the neck is turned into 

 the vesicle, within which the filament is coiled up. The remains 

 of the cnidoblast persist as a layer of protoplasm surrounding the 

 vesicle and produced externally into a slender sensitive process, 

 the cnidocil, which slopes towards the free end of the body or 

 tentacle, as the case may be. There are also other much smaller 

 nematocysts with shorter, thicker threads, and without barbs. 

 Small star-shaped nerve-cells, connected with the protoplasmic 

 investment of the thread-cells, are said to exist in the ectoderm. 



The endoderm (en) is made up of a single layer of large, irre- 

 gular, granular cells, each containing a big flattened nucleus with 

 nucleolus, and one or more vacuoles, often of great size. The 

 endoderm cells abut at one end upon the supporting lamella (and 

 appear to be here produced into transversely-wrsiRged tails), while 

 the other end projects into the digestive cavity, and can be thrust 

 out into pseudopodia. This end often bears one or more elon- 

 gated protoplasmic threads (flagella*) (/), which are capable of 

 being withdrawn, like pseudopodia. The external half of each 

 cell contains in Hydra viridis numerous small globular bodies, 

 coated with chlorophyll. Hydra fusca possesses small bodies of 

 similar nature, but containing particles. 



Reproductive Organs. Male and female sex-organs are differ- 

 entiated, and, as both occur in the same animal, Hydra is said to 



* Flagella are longer than cilia, capable of more complex movement, and 

 either occur singly or associated in small numbers. 



