LOWER ANIMALS 



205 



flatworms are very similar to the modern coelenterate 

 (especially hydromedusan) larval stages. Moreover, 

 in some respects it is as difficult to establish the 

 ancestry of all modern flatworms from an ancestral 

 flatworm. 



The ribbonworms are also believed to be offshoots 

 of an ancestral free-living flatworm — one that was 

 structurally more complex than the ancestor of all 

 flatworms, however. 



It is further assumed that ribbonworms have not 

 given rise to anything other than more complex 

 ribbonworms. In this respect they might, now at 

 least, be considered evolutionary "dead ends," as 

 could the mesozoans, sponges, and comb jellies. On 

 the other hand, the mere fact that these phyla have 

 not been successful in producing new phyla is no 

 reason to consider them dead ends. All have evolved 

 considerably, even if they did no more than retain 

 their phylum identities. Although none seems to 

 have produced a new phylum and no animal phyla 

 likely arose after Cambrian times, it is not neces- 

 sarily impossible for acoelomates (those phyla with 

 no body cavity) to evolve further in the future. It 

 must be remembered that one of the main character- 

 istics of the planet Earth is change — change in land 

 forms, climates, and animal habitats. Along with 

 these environmental changes, organisms either 

 change or perish. And the fossil record indicates that 

 many more animals perished than were fortunate 

 enough to acquire chance hereditary changes that 

 enabled them to survive. For these reasons, we cart 

 consider no living phylum an evolutionary dead end. 

 If such had ever been the case, the phylum would no 

 longer be here. 



PLATYHELMINTHES (Flatworms) 



Diagnosis: symmetry bilateral; body unseg- 

 mented, soft and usually much flattened from top to 

 bottom; worm-like; no body cavity, anus, circulatory, 

 respiratory, or skeletal systems; includes the free- 

 living flatworms of fresh and salt water and moist 

 terrestrial environments, and the parasitic tapeworms 

 and flukes. 



The flatworms are the first phylum to be discussed 

 whose activity is based on the possession of organs 

 arranged in organ systems. However, in many re- 

 spects a tissue-organ structure may better sum- 

 marize their organization. The appearance of organs 



is associated with the presence in developmental 

 stages of three germ layers, or embryonic cell groups, 

 from which the adult organism ultimately forms. 

 These germ layers are the outer ectoderm which 

 forms the skin and nervous system, the inner endo- 

 derm which forms the lining of the alimentary canal 

 and associated organs, and the middle mesoderm 

 which forms muscular, connective, and circulatory 

 tissues. In tissue-grade animals, coelenterates and 

 ctenophores, the cells in the "jelly" between the 

 outer and inner layers probably never form a com- 

 plete layer, hence there is no third germ layer. The 

 ctenophores may possess the third, or mesodermal, 

 layer but their development is sufficiently confusing 

 to leave the number of germ layers present in doubt. 



The Platyhelminthes possess three germ layers, 

 no body cavity, and are bilaterally symmetrical. Cir- 

 culation and digestion are carried on by a gastro- 

 vascular cavity similar to that found in the Radiata, 

 the coelenterates and ctenophores. The muscular 

 system in flatworms is composed of two muscle layers 

 in the body wall. Longitudinal nerve fibers with 

 cross-connections form a so-called "ladder type" of 

 nervous system. The animals usually are hermaph- 

 roditic (both sexes combined in one animal), with 

 both sexual and asexual means of reproduction. 



CLASS TURBELLARIA (Free-living Flatworms) 



Diagnosis: regularly free-living; most under 1/5 

 inch in length but free-living carnivores reach 20 

 inches; also commensals or parasites; body un- 

 divided; covered by epidermis with cilia, no cuticle or 

 suckers; mouth near middle of the undersurface; 

 mostly hermaphroditic, with direct development of 

 the adult form from the egg; also asexual by constric- 

 tion across the worm's body (Figure 12.9). 



The free-living flatworms inhabit fresh or salt 

 water, and even moist terrestrial situations. Some 

 few live either within or upon other animals. Many 

 are brilliantly colored; in some a yellow or green color 

 is due to the protistans or algae living in the tissues of 

 the worm. The life history is usually somewhat sim- 

 ple. The fertilized egg gives rise to an immature 

 form which transforms by simple growth and dif- 

 ferentiation to an adult. Asexual reproduction by 

 fragmentation and regeneration of each fragment is 

 known in many species. Although, regeneration is 

 highly developed and can play a part in reproduc- 



