204 



MESOZOA THROUGH ENTOPROCTA: 



Figure 12.8 Ctenophore types: 1, P/eurobroch/o {about natural size); 

 2, Beroe (about usual size, but grows to 8 inches in length); 3, Bo/inopus 

 (to about 2 inches long), a lobed and flattened genus; 4, Ces/um, Venus's 

 girdle (usually about 6 inches long, but grows to 3 feet). (Modified from 

 various sources.) 



Comb jellies may be confused with jellyfish, but 

 the above distinctions should be sufficient for 

 diagnosis. 



The comb jellies have the tissue basis of organiza- 

 tion seen in the coelenterates, but with modifications 

 and complications. The ctenophores resemble 

 coelenterates in the general appearance of their diges- 

 tive structure and jelly-like middle layer, and in the 

 lack of organ systems. They differ from the coelenter- 

 ates in lacking stinging and related cells (nemato- 

 cysts); in showing only one generation; and in hav- 

 ing muscle-like fibers in the middle layer, a 

 specialized sensory body, and a different type of 

 larvae. Despite these diflferences, it seems likely that 

 the comb jellies were derived from an ancestral 

 hydromedusan. 



The name of this phylum comes from eight longi- 

 tudinal rows of comb plates which are modified cilia 

 (see Ciliophora, p. 135) and are used like cilia for 

 locomotion. Beneath each comb-plate row is a nerve- 



like cord integrated into a general nerve net similar 

 to that of coelenterates. The ctenophores foreshadow 

 organ system organization by possessing reproduc- 

 tive ducts through which eggs or sperm are shed. 



Two anal pores on the top surface, which is op- 

 posite the mouth, as well as other features make 

 most ctenophores biradial in their symmetry. A 

 few species are creeping and bilateral, the general 

 body shape being ribbon-like. Earlier workers con- 

 sidered these animals to be similar to the ancestors of 

 the Platyhelminthes, but now this is not considered 

 likely. As was indicated previously, it is now usually 

 believed that the hydromedusans most closely re- 

 semble the Eumetazoan ancestor. 



ACOELOMATA: PLATYHELMINTHES 

 AND NEMERTEA 



All of the remaining phyla in the animal kingdom 

 possess organs that in turn are grouped into organ 

 systems. However, not all organisms have the same 

 number of organ systems. In general, the so-called 

 lower phyla have fewer and simpler organ systems 

 than the so-called higher phyla. In a sense it is in- 

 accurate to speak of "lower" and "higher" phyla 

 because both groups accomplish life processes suc- 

 cessfully, and no one knows which group does so 

 better. As a schematic device, however, such a dis- 

 tinction helps in charting the probable path of or- 

 ganic evolution and points up the fact that increased 

 structural complexity was a major characteristic in 

 the development of animal diversity. 



The tendency for greater complexity can be traced 

 from a hypothetical stem jellyfish to development of 

 animals above the level of tissue organization. This 

 tracing also serves to clarify some misconceptions 

 that may have arisen from previous discussion of 

 evolution. 



Early evolution from stem jellyfish to flatworms is 

 believed to have been through an early hydromedusan 

 larva, not the adult jellyfish stage. (This creates no 

 particular problem. In certain living animals the 

 larva may reproduce; it may even be sexually mature 

 and reproduce sexually. In such cases adults may or 

 may not occur.) This step is thought to be from 

 coelenterate larva to a stem, or ancestral, free-living 

 flatworm. Although such a step might seem to be 

 quite great as far as structural change is concerned, it 

 is not. Even certain living larvae of the free-living 



