small jellyfishes, except that they may be 

 of different shapes. In fact, formerly the 

 ctenophores (pronounced — ten-o-fores) were 

 classified as coelenterates. There are, how- 

 ever, a number of important differences, and 

 these seem to justify placing the "comb jel- 

 lies" in a separate phylum, the Ctenophora. 



Distinguishing Features. Ctenophores com- 

 monly are called comb jellies, because each 

 displays eight radially arranged rows of cilia, 

 called "combs" (Fig. 32-12). The cilia permit 

 these floating animals to move very slowly 

 through the water. But ctenophores are very 

 feeble swimmers. Great numbers of them 

 often drift together in the tides; and some- 

 times many are thrown up on the beach 

 when a strong onshore wind is blowing. 



Many Ctenophora are also known as sea 

 walnuts, because they are about the size and 

 shape of the common walnut. In the sea, the 

 comb jellies appear as transparent ghosts of 

 rare and delicate beauty. By day they show 

 iridescent hues of blue and rose; and at 

 night they emit a faintly glowing light from 

 eight rows of luminescent cells, along the 

 bases of the combs. 



TENTACLE 



CILIARY 

 COMB 



GASTROVASCULAR 

 CAVITY 



MOUTH 



Fig. 32-12. One of the comb jellies: Phylum Cteno- 

 phora. Note the combs. These animals show iridescent 

 colors by day and a faint luminescence by night. 



The Animal Kingdom - 635 



Ctenophora and Coelenterata resemble 

 each other in several respects. Both have a 

 saccular (rather than tubular) enteron in 

 which the mouth opening serves for egestion 

 as well as for ingestion; both have an abun- 

 dant, usually gelatinous, mesoglea separating 

 the outer ectodermal and the inner endo- 

 dermal cell layers. However, the mesoglea ol 

 the ctenophores contains well-developed, 

 though scattered, muscle fibers and very nu- 

 merous amoebocytes. Many biologists, there- 

 fore, consider that the Ctenophora possess a 

 primitive mesodermal layer, which would 

 identify them as triploblastic animals. Cteno- 

 phores have only two tentacles and no nema- 

 toblasts; and all species are hermaphroditic. 

 Moreover, the symmetry, while basically 

 radial, shows some tendency toward bilater- 

 ality in the placement of some structures. 

 And finally, a greater tendency toward the 

 development of definite organs is found 

 among the Ctenophora. Most species possess 

 an equilibrium organ, or statocyst (similar 

 to the one depicted in Fig. 23-7) with nerve 

 connections leading to the muscle fibers; and 

 usually there are definite ducts which con- 

 duct the eggs and sperm from the internally 

 placed gonads to the outside. 



Ctenophora do not produce any hard skele- 

 tal materials and consequently a fossil record 

 of ancient forms is virtually lacking. How- 

 ever, their general similarity to the Coelen- 

 terata, both as to structure and development, 

 leads to the belief that the ctenophore stock 

 was very close to the direct evolutionary line 

 that led to the development of higher ani- 

 mals. 



THE PLATYHELMINTHES (LITERALLY, 

 FLATWORMS) 



Although it includes only about 7000 spe- 

 cies, this phylum is a very important one. 

 Some of the flatworms are small free-living 

 animals such as Planaria (Fig. 16-7). These 

 live mainly in quiet waters (both fresh and 

 salt), although there are a few land species, 

 which live in moist, swampy places. But a 



