300 ELEMENTS OF COMPARATIVE ZOOLOGY. 



globular outgrowths, medusa-buds. Sketch a hydranth en- 

 larged, showing the points made out. 



Study a mounted specimen under higher microscopic 

 powers, and see that the zooids are made up of two layers 

 and that they contain a central digestive cavity. Can you 

 trace the layers and the cavity into the hydrocaulus ? 

 Could food taken into one of the hydranths pass to another 

 hydranth ? Are the tentacles solid or hollow ? Examine 

 the tip of a tentacle of the series nearest the mouth, and 

 see the large oval nettle-cells imbedded in it. (In favor- 

 ble specimens threads can be seen extending from the net- 

 tle-cells). Sketch a hydranth enlarged, showing layers, 

 digestive tract, etc., and a medusa-bud. 



Look carefully over the hydranths and see if you can find 

 any traces of an oesophagus turned into the body as in the 

 sea-anemone ; of septa, and of mesenterial filaments. Do 

 any individuals show a bilateral nature ? 



COMPARISONS. 



With columns for Sea-anemone and for Hydroid, answer 

 the following questions : 



(1) Are the animals simple or do they form colonies con- 

 nected tegether ? 



(2) Can you find traces of bilaterality in the animals ? 



(3) Are septa present ? Is the digestive cavity simple, 

 or is it subdivided into chambers ? 



(4) Is there an inturned oesophagus ? 



(5) Are the tentacles hollow or solid ? 



