150 



Obelia 



THE RISE OF ANIMAL LIFE 



This tiny colonial coelenterate may be 

 found attached to seaweed and other ob- 

 jects lying in the clear water of tide pools 

 and below low tide level to a depth of 

 several fathoms. It is attached to the sub- 

 stratum by means of a horizontal branching 

 basal portion. 



An obelia colony begins as a single polyp 

 which by budding and the subsequent 

 clinging together of the buds forms a col- 

 ony (Fig. 8-13). The process is reminiscent 

 of the bud-upon-a-bud condition observed 

 in hydra. The tissues and gastrovascular 

 cavity are thus continuous throughout the 

 colony. There are two types of polyps in 

 an obelia colony, the feeding polyp, or 

 hydranth, and the less common reproduc- 

 tive polyp, or gonangium. The hydranth is 

 not greatly different from hydra except that 

 it possesses solid instead of hollow tentacles 

 and it is surrounded by a tough, horny 



outer covering called the perisarc, which 

 invests the entire colony. The transparent 

 vase-like portion of the perisarc surround- 

 ing a hydranth is called the hydrotheca. 

 The cellular portion just beneath the peri- 

 sarc is known as the coenosarc. After food 

 has been captured and partly digested by 

 an individual hydranth, it is carried through 

 the gastrovascular cavity by the beating 

 flagella which line the cavity. Thus all 

 polyps share in the good fortune of any 

 one. Digestion is finally completed intra- 

 cellularly in the lining cells. 



Obelia reproduces asexually by forming 

 buds either on the horizontal parent stalk 

 or on the upright stalk. Sexual reproduction 

 occurs in a second type of individual, the 

 gonangia (singular, gonangium), which 

 have no tentacles and no mouth. This cylin- 

 drical polyp is covered by the transparent 

 gonotheca, a continuation of the perisarc. 

 Each gonangium contains a central stalk, 

 the blastostyle, upon which are borne small 



batteries of 

 nematocysts on 



tentacle 



tentacular bulb 

 Njelum 



tentacle 

 statocyst 



Suctorial 



umbrella 



noesoqioea 



— radial canal 



qastrovascular 

 ^ covity 



qonad 



— manubrium 



mouth 



circular cana] 



r- o lA i~ «,»-^„=-4 ♦« cl,ow mtornal structure. Also the tip of one tentacle is enlarged. 



Fig. 8-14. Gonionemos sectioned to snow iniernai siru».i«i«s. i- 



