only one to two feet); and (4) Echinococcus 

 granulosus, the hydatid worm (adults, less 

 than one inch long, in dogs, wolves, and other 

 carnivores; huge multiple larval cysts, called 

 hydatid cysts, up to two-inch diameters, 

 found in the brain and other organs of man, 

 monkeys, and various domestic animals; cysts 

 may inflict serious damage or even death 

 upon the host; about 500 human cases re- 

 ported in United States to date). 



Phylogeny. Not much definite information 

 is available in regard to the evolutionary ori- 

 gin of the Platyhelminthes. None possesses 

 any skeletal parts and virtually no fossil 

 flatworms have been found in the earlier 

 geological strata. However, judging from 

 other evidence — the symmetry, mesodermal 

 organization, embryological development, 

 and so forth — it seems probable that the 

 early flatworms initially followed the main 

 line that led to the evolution of most of the 

 higher animals (Fig. 29-11), before divergence 

 occurred. 



FIVE PROBLEMATICAL PHYLA 



There are a number of lowly invertebrate 

 groups, 3 generally considered as phyla, among 

 which the evolutionary relationships are 

 quite obscure. Five of these — the Nemer- 

 tinea, Nemathelminthes, Rotifera, Bryozoa, 

 and Brachiopoda — are relatively more im- 

 portant. These will be considered in very 

 brief fashion, more or less collectively. 



All representatives of the foregoing groups 

 are bilaterally symmetrical. But even more 

 significantly, all possess a tubular type of 

 enteron. This represents an important ad- 

 vance that becomes established as a standard 

 feature in higher animals generally. Some of 

 these five groups show the first appearance of 



3 In addition to the ones mentioned above, there 

 are quite a few other small and obscure groups, for 

 which no generally accepted classification has been 

 worked out. These include: the Mesozoa, Entoprocta, 

 Gastrotricha, Kinorhyncha, Nematomorpha, Acantho- 

 cephala, Phoronidea, Chaetognatha, Sipunculoidea, 

 Priapuloidea, Echiuroidea, and, perhaps, one or two 

 others. 



The Animal Kingdom - 641 



a true body cavity, or coelom, and the first 

 clearly defined blood vessels— two other fea- 

 tures that are standard in all higher forms. 



The coelom is a mesodermal cavity. It is 

 important because it separates the body wall 

 from the wall of the enteron. This allows the 

 movements of the body to become independ- 

 ent of the movements of the digestive tract 

 (see discussion, page 279). Moreover, the 

 tubular enteron allows the digesting food to 

 follow a "one-way traffic" route; and this in 

 turn permits the successive parts of the diges- 

 tive tract to become differentiated and spe- 

 cialized in a more efficient manner. Each part 

 of the tube can become uniquely designed to 

 perform some particular part of the work of 

 digestion and absorption, as in the digestive 

 tract of man (Fig. 16-11) and of other higher 

 animals. 



The Nemertinea. These animals (Fig. 32- 

 18), which commonly are called ribbon 

 worms or proboscis worms, are represented 

 by about 500 species, mainly marine, with 

 soft, flat, unsegmented bodies. Sometimes 

 they are white, but more often they are col- 

 ored (red, brown, green, or yellow). Usually 

 the nemertines are only a few inches long; 

 but there is one species that can reach a 

 length of 75 feet. 



Sometimes the nemertines are grouped 

 with the flatworms. The mesoderm consists 

 mainly of loosely arranged tissue in which 

 no definite coelomic cavity is present, al- 

 though there are some flame cells. Unlike 

 the flatworms, however, nemertines have a 

 tubular digestive tract; and there are three 

 poorly differentiated "blood vessels" running 

 lengthwise through the mesoderm (Fig. 32- 

 18). Characteristically, the food-catching ap- 

 paratus is an elongate muscular proboscis, 

 sometimes tipped with a sharply pointed 

 stylet, which the animal forcibly throws out 

 from its anterior end (Fig. 32-18). 



The Rotifera (Fig. 32-19). Most of these 

 "wheel animals" are of microscopic size. In 

 fact they are scarcely larger than most Pro- 

 tozoa, among which they often live. Many 

 different species can usually be found in quiet 



