50 COELENTERATA 



5. The gonads hang from beneath the radial canals into 

 the subumbrellar space. They are lobulated in structure, 

 and more or less prominent according to maturity and the 

 breeding season. The eggs or spermatozoa, as the case may 

 be, are dehisced from these into the water directly. 



During the breeding season specimens placed in the dark 

 in the latter part of the afternoon and left for two or three 

 hours will shed eggs and sperm. The fertilized egg under- 

 goes cleavage, a planula is formed that finally attaches at 

 one end and develops into the hydra stage. Eggs are nor- 

 mally laid about 8 p. m. 



6. The tentacles. Is their arrangement a radially sym- 

 metrical one? How are the nematocysts arranged on them? 

 Look for adhesive organs on them. Of what use are such 



organs? 



Turn your specimen with the velum side toward you and 

 study the edge of the medusa with a low-power objective for 

 the sense organs. These are of two kinds: 



(a) The larger, round bodies at the bases of the tentacles 

 communicate with the circular canal (which may possibly be 

 seen along the edge of the bell) . They are filled with a layer 

 of strongly pigmented endoderm cells and are probably light- 

 percipient organs. 



{b) Other small sessile and transparent outgrowths, situ- 

 ated between the bases of the tentacles, are the so-called 

 statocysts (lithocysts) , which are probably static organs. 



All of the tentacles are abundantly supplied with tactile, 

 sensory cells. There is a well-established circumvelar nerve 

 ring (not easily determined in living material) derived from 

 the ectoderm, also scattering nerve cells beneath the ecto- 

 derm in connection with the muscular tissue. Exumbrellar 

 and subumbrellar layers of muscle fibers are also present. 



Make a drawing from the side, slightly tipped, to show 

 the velum, and another as seen from the oral surface. 



Brooks: Life History of Hydromedusae. Mem. Bost. Soc. Nat. Hist., 3, 

 1886. 



