1^8 THE ANIMAL KINGDOM 



the several senses scattered around the umbrella margin. Notable excep- 

 tions are tlie nerves from the statocysts, which go to the motor ring. 

 While the other senses influence die activity of the medusa, and may 

 even reverse its direction of movement, the direction itself is related 

 only to gravity. The intimate association of the gravity sense with the 

 motor ring is therefore significant. 



Locomotion is efficient only when the muscle fibers of the umbrella 

 contract synchronously. Coordination is effected by the circular fibers of 

 the nerve ring. If the ring is cut, coordination is lost and the medusa 

 swims erratically with lopsided beats. 



Feeding behavior requires coordination oriented radially rather 

 than circularly. The shortening and bending of the tentacles, the bend- 

 ing of the umbrella, and the lateral bending of the manubrium must 

 be in the right direction if the prey is to be successfully transferred to 

 the mouth. The nerve ring is not important in this behavior and may 

 be cut without serious eftect if the cut is not exactly on the radius 

 involved. The radial nerve fibers of the subumbrellar surface are in- 

 volved, for this coordination disappears if they are cut. The manubrium 

 may extend and bend, but fails to bend in the right direction. 



64. Gonionemus: Reproduction 



Both male and female Gonionemus have four gonads that develop 

 in the epidermis of the subumbrellar surface and hang downward as 

 ruffles parallel to the four radial canals (Fig. 10.2). Since the canals are 

 close to the subumbrellar surface the gonads are close to a nutrient 

 source. Eggs and sperm are shed into the surrounding water where fer- 

 tilization takes place. 



The fertilized egg develops rapidly into a small ciliated larva, the 

 planula (Fig. 10.7, A). The planula is a swimming gastrula composed of 

 a layer of ectoderm enclosing a solid core of large endoderm cells. 

 Planulae are found in all of the classes of coelenterates. The planula of 

 Gonionemus does not develop directly into a medusa, but attaches to 

 some solid object and becomes a polyp (Fig. 10.7, B). 



The polyp is tube-shaped with an outer epidermis and inner gas- 

 trodermis separated by a very thin mesoglea. The tube is closed at the 

 attached end, forming a foot, and the open free end is the mouth. The 

 simple cylindrical cavity is the stomach. Surrounding the mouth is a 

 ring of tentacles bearing nematocysts. Like the medusa, the polyp feeds 

 by snaring prey Avith its outstretched tentacles and transferring it to the 

 extensible mouth. 



Structurally the polyp is simpler than the medusa. Circular and 

 longitudinal muscle fibers are sparse and not arranged in layers or sheets 

 as in the medusa. The nervous system lacks a nerve ring, and throughout 

 its structure suggests a nerve net with neurons somewhat more numerous 

 around the mouth. In many respects the polyp is a juvenile stage, inter- 

 mediate between the planula and the medusa. 



The polyp of Gonionemus, only 1 mm. in diameter, is unusually 

 small and squat. As it grows, the polyp reproduces asexually by budding. 



