HYDROZOA. 21 



larger thread-cells, which are fewer in number, 

 are further distinguished by the possession of a 

 sheath, surrounded b}^ three recurved barbs, and 

 terminating in a long slender thread (c). The 

 length of the Hydra, exclusive of its tentacles, 

 seldom exceeds three fourths of an inch. 



More minutely examined, the body of the Hydra 

 is found to be composed of two membranes, an 

 ectoderm and an endoderm. The former consti- 

 tutes the outer layer of the animal, and has one 

 of its sides always exposed to the water wherein 

 the Hydra lives, the other side being in rather 

 close contact with the endoderm, whose free surface 

 forms the lining of the large internal cavity. The 

 tentacles, which open into this cavity, are tu- 

 bular prolongations of both the above membranes. 

 It has been asserted by Trembley that the body of 

 Hydra may be turned inside out, without thereby 

 sustaining any injury, or being checked in the 

 performance of its proper functions ; but this 

 experiment needs repetition. Both ectoderm and 

 endoderm are vacuolated, especially the latter, and 

 hence the well-known granular appearance which 

 the Hydra presents under the microscope. Some 

 describe the endodermal lining as produced into 

 a number of villous elevations, projecting into the 

 digestive cavity, and placed at right angles to its 

 axis. The thread-cells are chiefly developed in 

 connection with the ectoderm, and the numerous 

 nodules, filled with these bodies, which the ten- 

 tacles exhibit are merely enlargements of this 

 outer layer. The ectoderm of the tentacles (and 

 perhaps, also, of the body-wall) shows slight traces 

 of the presence of muscular fibres. Around the 

 margin of the mouth, the ectoderm and the endo- 

 c 3 



