238 '■"f ANIMAL KINGDOM 



The asdielminthes are difficult to relate to any of the other groups. 

 The degree to vvhicii their cells are specialized gives them a ditferent 

 appearance. Some zoologists believe they are derived from a fiatworm 

 type by the separation of mouth and anus and the addition of a pseudo- 

 coeloni. Although the aschelminthes are simpler than the higher in- 

 vertebrates in tne sense that they lack both a circulatory system and 

 muscles around the gut, they are complex from the point of view of such 

 features as cell constancy and cellular differentiation. 



101 . The Evolution of the Coelom 



The major groups of higher invertebrates, including the molluscs, 

 annelids, arthropods, echinoderms and chordates, all have a separate 

 mouth and anus, a muscular gut, a true coelom and a well developed 

 circulatory system. In some of the minor groups one or another char- 

 acter is absent, but such cases are believed to represent losses during 

 their evolution from ancestors in which the characters were present. 



The distinctive characteristic of these animals is the coelom (or 

 eucoelom), a cavity within the mesoderm lined with a delicate epi- 

 thelium, the peritoneum. These phyla are often grouped together as the 

 Eucoelomata. 



The coelom may appear during development by either of two 

 methods, depending on the species. The mesoderm may form first as 

 solid masses and the coelom later by cavitation within the mesoderm 

 (Fig. 13.1). Such a coelom is a schizocoelom (cavity by splitting). In 

 other eucoelomates the mesoderm and coelom are formed together as 

 pouches from the original gut cavity of the gastrula (Fig. 13.1); the wall 

 becomes the mesoderm and the separated cavity persists as the coelom. 

 Such a coelom is an enterocoelom (cavity from the gut or enteron). In 

 both methods the coelom usually appears first as one or more pairs of 

 cavities beside the digestive tract. The result is similar, regardless of 

 method of origin. The paired cavities are usually enlarged until they 

 meet above and below the gut, where the two lining epithelia come to- 

 gether and often persist as a supporting membrane, a mesentery. 



In general the molluscs, annelids and arthropods are schizocoelous, 

 whereas the echinoderms, hemichordates and lower chordates are entero- 

 coeious. Many students believe that these two methods of coelom forma- 

 tion are basically different, and that the eucoelomates should be divided 

 into two groups, the Schizocoelomata and the Enterocoelomata, im- 

 plying that they arose independently from noncoelomate ancestors. At 

 the present time, however, a number of exceptions are known (the 

 arthropod housefly and tardigrades are enterocoelous, higher chordates 

 are schizocoelous, both kinds of development are found in the small 

 phylum Brachiopoda, etc.) which suggest that the difference is not 

 really basic, and that one kind of development may easily have evolved 

 from the other. This is consistent with the observation that, aside from 

 the method of origin, the schizocoelom and enterocoelom cannot be 

 distinguished. 



The basic excretory organ of the lower invertebrates is the 



