INVERTEBRATE ZOOLOGY. 69 



30. [Tltis difficult experiment is placed here only as one irJiicli may be 

 done, and should he attempted only by patient iiwestigators. n-Jio are prepared 

 for repeated failures.] Trembly, a French zoologist, in 1744. cut livina: hydras 

 in two and found that under favorable conditions each piece would grow 

 again into a perfect hydra. He even cut one into several pieces and found the 

 larger portions were capable of becoming comi^lete animals. 



This experiment is easy to attempt, but results are hard to obtain. Cut 

 several hydras in different ways — transverse, longitudinally, etc.. and keep 

 the portions in a beaker of clear water. Scissors may be iised for this, but a 

 sharp razor is better. 



31 • Study of a cross-section. To kill a hydra in an expanded condition, 

 place in a watch crj'stal with a very little water, and suddenly deluge the ani- 

 mal while expanded with a test tube full of hot alcoholic corrosive sublimate 

 [70^ + HgCl,]. Allow it to stand a few minutes until cool and then place in 

 a large amount of pure 70 >;. which should l)e changed once after a few hours. 

 Such a specimen may be treated as the Sycon Sponge in 22 and sectioned. 

 In the section, notice the two layers of cells, ectoderm and endoderm, separa- 

 ted by a definite line, the supporting lamella. Sections of different animals 

 and of the same animal at different planes, show different points, as follows : 



a) Ora. large cells with amoel)oid processes, crowded in between the two 

 layers, but belonging to the ectoderm. Sections at aboiit the middle of the 

 body show this. 



b) Testes, a conical mass of cells, belonging to the ectodermic layer, form- 

 ing a protuberance. Mostly in the upper portion of the vase-shaped body. 



c) Cross-section of a tentacle. Notice the few huge endoderm cells which 

 form a lining and enclose a minute central lumen continuous with the gas- 

 trocoele. 



d) Nettle cells, {nematocysts). These may be found anywhere, even in endo- 

 derm. but are most common at the ends of the tentacles. They are large, 

 oval capsules, semi-transparent, and unich more refractive than the surround- 

 ing tissue. 



Type IV. -Campanularia. 



32. Make preliminary examinati(jn of material in a dish of 70'/,. No- 

 tice the two forms of polyp, each enclosed in a ti-ansparent cup. Select and 

 cut off with scissors one or two good branches for mounting. These should 

 include specimens of both sorts of polyps : and. if possible, in different stages 

 6 



