20 PENNARIA TIARELLA. 



Technical Note. With a pair of fine scissors snip off one of the hydranths, 

 leaving a short piece of the stem attached, and mount it in water on a 

 glass slide. Examine with a low power of the compound microscope. 



THE HYDRANTH. Note the flower-like form of an expanded 

 hydranth. In the center is the body, shaped somewhat like a tenpin or 

 Indian club. Encircling its base is a whorl of long slender tentacles 

 and nearer the distal end are others of a different shape. Can you 

 make out the number of each kind of tentacles ? Is the number con- 

 stant ? Are the short tentacles in a single whorl ? 



In addition to the tentacles, certain large ovoid bodies may be seen to 

 spring from just above the whorl of long tentacles in some of the 

 hydranths. These are the medusa-buds. In some species of Pennaria 

 they become detached and swim about; in other species they remain 

 attached to the hydranth. In the latter case they are called sporosacs. 



The knob-like distal end of the body is termed the manubrium. At 

 its extremity is the mouth, an aperture capable of some dilation and 

 contraction. It will probably not be seen in the preserved specimen. 

 The mouth opens into the cavity of the manubrium and body, both of 

 which are hollow. This hollow space is the common digestive and circu- 

 latory cavity of the animal, and is known as the enteron or gastro-vas- 

 cular space. The ccenosarc being also hollow, its tubular cavity is 

 continuous with the cavity of the hydranth and forms a part of the 

 gastro-vascular space. Circulation from one hydranth to another is 

 thus established. 



It will be noted that the ccenosarc is in most places separated by an 

 interval from the perisarc, but that processes of the former extend out 

 toward the latter at irregular intervals. 



Exercise 2. Draw a hydranth, giving attention to detail not shown clearly 

 in the -first exercise. 



THE TENTACLES. Under a higher power of the microscope it will be 

 seen that a tentacle is made up of an axis of large endoderm cells sur- 

 rounded by a layer of small ectoderm cells. Between these is the 

 delicate non-cellular mesogkea, corresponding to the mesodenn in 



