THE PHYLA COELENTERATA AND CTENOPHORA 201 



The processes of feeding and digestion are similar to those found 

 in the coelenterates. 



The sensory region of the comb jelly is concentrated at the aboral 

 end where a statocyst is the primary sense organ. As in medusae, the 

 statocyst is associated directly with motor nerves which, in the cteno- 

 phore, are the eight radiating nerves underlying the comb columns. If 

 one of these nerves is cut, the corresponding column is no longer co- 

 ordinated with the others. If all the neives are cut, coordination disap- 

 pears and the comb jelly is unable to control its locomotion. 



Like most jellyfish the ctenophores are transparent. The combs, 

 however, reflect light and produce iridescent patterns. This shimmering 

 color passing in waves from aboral to oral ends shows the waves of beat- 

 ing of the combs and is useful in studies of coordination. 



At night many of the comb jellies are brightly luminescent when 

 disturbed. The light is produced close to the nerve tracts beneath the 

 comb columns. Like luminescence in most animals the light is blue-green 

 in color. In ctenophores the luminescence is especially striking since it 

 becomes irridescent as it is reflected from the combs, flickering like 

 colored fire up and down the comb columns. 



The eggs and sperm are shed into the water where the embryos 

 develop directly into the comb jelly form. Early divisions of the eggs 

 follow an exact, rigid path of development. The first three divisions are 

 vertical and produce a curved plate of eight cells. The fourth is hori- 

 zontal and separates eight small upper cells from eight large lower cells. 

 Later cleavages continue to be constant in all individuals, and each 

 upper and lower cell of the 16-cell stage becomes the corresponding 

 eighth of the ctenophore. Associated with this rigid pattern is an early 

 chemical differentiation. The opposite is true of most coelenterates, in 

 which early development appears to be unspecialized, with cell division 

 preceding chemical differentiation. 



71. The Regulation of Form 



A fascinating and challenging area of biology is concerned with two 

 problems associated with development: the extent to which an organism 

 can repair injuries, and the extent to which it can correct disarrange- 

 ments. Some of the coelenterates and ctenophores have remarkable 

 abilities in these respects. If parts of the body are removed, they are 

 usually replaced. If individuals are cut in half, each half may regenerate 

 the missing half. Sometimes quarters or even smaller pieces of animals 

 will regenerate into Avhole organisms. 



Very often a remarkable regenerative ability is associated with 

 natural reproduction by budding. In many sea anemones, for example, 

 pieces of the base may break off spontaneously and develop into new 

 individuals. Hence, when pieces are cut off, they regenerate well. Again, 

 in the one genus of sea anemones in which a single, experimentally re- 

 moved tentacle can regenerate all the missing parts, it is found that 

 tentacles spontaneously do the same thing as a form of asexual repro- 

 duction. 



