34 CCELENTERATA. 



3. The short but rather large body of the hydranth passes 

 back to the perisarc as the fleshy axis, cceiiosarc. 



4. Notice the gonosomes between the rows of tentacles. 

 What is their origin and arrangement? This is a form in which 

 the medusae are not set free, but remain vestigial. They show 

 neither radiating nor circular canals. The gonads ripen on the 

 partially developed manubrium of the medusa. The sexes are 

 separate. 



Make a drawing of a hydranth. 



5. The male gonophores when nearly mature are rounded or 

 elongated with the space apparently between the manubrium 

 and sub-umbrellar surface filled with sperm. In fact, the sperm 

 are enclosed between the ectoderm and endoderm of the 

 manubrium, but the ectoderm is pressed over against the ecto- 

 derm of the sub-umbrella so this space is practically obliterated. 

 These sperm become active when liberated in sea-water. 



6. The female gonophore when mature is more elongated, 

 shows indications of tentacles at the free extremity, and there is 

 an actual sub-umbrellar space. The eggs are formed in the 

 ectoderm of the manubrium, are shed into the sub-um])rellar 

 cavity, and develop into actimdcB. With needles open a female 

 gonophore and examine the developmental stages, (a) Some- 

 what irregular disc-shaped embryos with a variable number of 

 projections around the margin, the forming tentacles (6). 

 Older stages with the tentacles more developed and with disc- 

 or lens-shaped bodies in which the coelenteric cavity can be 

 easily seen, (c) Actinula stage. Essentially a small polyp. 

 Notice the number of tentacles, the position of the mouth, and 

 the method of locomotion. 



Actinulse kept in a covered watch-glass of sea-water will 

 attach and form the basis of new colonies. 



Make drawings of gonosomes, gonangia, and developmental 

 stages. 



7. The arrangement of the attached medusae is best seen in 

 sections. 



