Phylum Annelida and Other Wormlike Coelomates 447 



molluscs. The vertebrates, too, have a true coelom, but its embryo- 

 logical origin is different from that of the annelids. In the development 

 of the annelid coelom, blocks of mesoderm split to form the coelom 

 lined with peritoneum. This type coelom is known as a schizocoel 

 since its arises by splitting. This contrasts with the vertebrate coelom 

 or enterocoel which arises as outpocketings or mesodermal sacs from 

 the roof of the archenteron. In the adult structure, these coeloms are 

 identical. 



Segmentation or metamerism as it first appears in the annelids 

 likewise has been successfully exploited by the higher animals including 

 the vertebrates and arthropods. By definition metamerism is the serial 

 repetition of sections from the anterior to the posterior end. Each small 

 segment or section contains representative portions of the various 

 organ systems. If all the segments are identical it is termed homon- 

 omous metamerism; if they are unlike, it is termed heteronomous me- 

 tamerism. No living animal is completely homonomous as the head 

 and anal segments are usually modified ; however, the annelids ap- 

 proach very closely to the basic homonomous type. 



Along with the development of true metamerism, paired appendages 

 appear among these forms. Among the annelids, primitively each 

 segment must have had a single pair of appendages, but most living 

 forms possess an increased number. The earthworm, Lumhricus ter- 

 restris, possesses four pairs of small rodlike setae on each segment, 

 while many of the marine annelids possess very elaborately developed 

 appendages. The use of these appendages for locomotion is another 

 feature coordinated with the flexibility attained by the metameric body. 



Although various other phyla have had organ systems, the an- 

 nelids are the first to show nearly all the systems (digestive, excre- 

 tory, reproductive, circulatory, etc.) in a single form. This again is 

 the typical condition found among the higher invertebrates and demon- 

 strates the great advances which the annelids have made over the 

 other worms. 



The body wall consists of several distinct layers. The outermost 

 of these is the thin noncellular cuticle which appears iridescent due to 

 the many striations on its surface. This possesses numerous .pores 

 through which the glands of the epidermis discharge their products. 

 The epidermis which is directly below the cuticle is a single-celled 

 layer; many of the cells are enlarged and function as unicellular 

 glands. Below this epidermis are the muscle layers, consisting of 



