Economic Importance of Animals 537 



PHYLUM 4— CTENOPHORA (COMB JELLIES OR 

 SEA WALNUTS) (Fig. 97) 



Ctenophores are found in warm and temperate seas where they eat 

 fish eggs, the larvae of Crustacea, oysters^, and other mollusks. Ctenoph- 

 ores are eaten by some marine animals. In the dark, they produce 

 and emit a luminescent light from beneath their comb plates. Because 

 of this phenomenon they are a source of much interest to the nocturnal 

 visitor to the seashore. 



PHYLUM 5— PLATYHELMINTHES (FLATWORMS) 



The adults and larvae of tapeworms found in the alimentary canals of 

 man and other animals interfere seriously with the digestion and ab- 

 sorption of foods (Figs. 182, 183, and 268). The larvae of certain dog 

 tapeworms (Echinococcus granulosus) may form large vesicles or blad- 

 derlike structures in man. These structures are known as hydatids or 

 hydatid cysts which may rupture with serious or fatal results. The larvae 

 of the dog tapeworm (Multiceps multiceps) (Fig. 268) cause "staggers" 

 or "gid" in sheep by lodging in the brain or spinal cord. Cattle, deer, 

 and goats also may be affected. The broad fish tapeworm (Diphyl- 

 lobothrium latum) may cause severe anemia in man. This form is trans- 

 mitted by improperly cooked fish. In general, the tapeworms (class, 

 Cestoda) are internal parasites usually present in the alimentary tract 

 and requiring an invertebrate or another vertebrate animal as their 

 secondary host. 



There are many parasitic flatworms in mammals, birds, and fishes, 

 although there is little danger of contracting disease if the meats are well 

 cooked. All flatworms of the class Trematoda are parasitic either in or 

 on the bodies of invertebrate or vertebrate animals. Fossils of flatworms 

 are commonly encountered, although thiey occur from the Pennsylvanian 

 epoch (255 million years ago) down to modern times (Figs. 320 to 322). 



Planaria (Figs. 177 to 179) are used frequently in experiments in re- 

 generation (Fig. 28) and grafting, for which they seem to be particularly 

 qualified. Dr. C. M. Child, of the University of Chicago, experimented 

 with Planaria and as a result elaborated his important Theory of Axiate 

 Organization of Animals. According to this theory, there is in all animals 

 a gradient of metabolic activity located along an imaginary axis. The 

 most active end of the gradient exercises a functional dominance over all 

 other lower regions. Planaria was used to illustrate a principle, which 

 in all probability will also apply to many, if not all, other animals. 



