24 GONIONEMUS VERTENS. 



forming the gastro-vascular space, a common digestive and 

 circulatory cavity. 



d) From the edge of the umbrella hangs a fringe of tentacles. Count 



the number in one of the quadrants intercepted by two of the 

 radial canals. They are much longer and more thread-like in the 

 living medusa and have the power to contract and stiffen when the 

 animal is irritated. 



e) Note the arrangement of the nematocysts on the tentacles. How 



does the grouping compare with that of the stinging thread cells in 

 Pennaria ? 



f) The concave sub-umbrella is partly closed in by a perforated dia- 



phragm, the velum. This muscular membrane is the principal 

 organ of locomotion. 



g) At the base of the tentacles are strongly pigmented round bodies, the 



sense organs. Their function is probably the perception of 

 light. 



h) The lobulated reproductive elements hang from the radial canals 

 into the sub-umbrellar space. They are more or less prominent 

 according to the conditions for breeding when the specimen was 

 taken. The two sexes are separate, but there is no noticeable 

 difference in the appearance of the elements. The egg- or sperm- 

 cells are shed out into the water. 



LOCOMOTION. The medusae are free-swimming animals, moving 

 with the convex surface directed forward by partly opening and closing 

 the umbrella. This expansion and contraction of the umbrella is in 

 turn secured by the action of the muscular velum. Sometimes, as the 

 medusa swims, the umbrella becomes turned inside out. 



Exercise i . Make one drawing of the medusa from the oral aspect and one 

 from the side, each on a scale of two. Also draw on a larger scale a 

 portion of the edge of the umbrella, including the bases of three or 

 four tentacles. 



Exercise 2. Mount the distal half of one of the more slender tentacles on a 

 slide and study with the compound microscope. Draw. 



