EMBRTOLOOT OF LEEGIIES. 339 



The early phases in the embryological development of the 

 leech {Olepsine) strongly resemble those of corresponding 

 stages in the vertebrates, according to a number of observers. 

 The origin of the germ-bands, the presence of the primitive 

 streak as well as the mode of cleavage, and the formation of 

 tlie gastrula* and neurula, show that, up to a comparatively 

 late period of embryonic life, some worms (Annulata) and 

 the Vertebrates travel along the same developmental path. 

 As observed by Whitman, the neurula of the chick, or of 

 the fish, belongs to the same type as that of Olejjsine. 

 Whether the Vertebrates ever descended from the worms or 

 any other type of Invertebrates or not, it is a matter of fact 

 that there is an essential unity in organization and mode of 

 early development in all the Metazoa, or three-germ-layercd 

 animals, and that the vertebrates are probably only a very 

 highly specialized group of animals, a branch of the same 

 genealogical tree from which have sprung the only less 

 generalized groups or branches of Mollusca, Annulata, and 

 Arthropoda. Certainly the division of the animal kingdom 

 into Vertebrates and Invertebrates, however useful, is essen- 

 tially artificial and misleading. Hence it follows that a 

 study of the Annulata, as well as other types of worms, must 

 prove to be fruitful in valuable results, and lead to what 

 may seem startling conclusions. 



Order 2. Annelides. — To this order belong the earth- 

 worm and sea-worms. The structure of the common earth- 

 worm {Lumlricus terrestris Linn., Fig. 151) is essentially 

 like that of the leech. Externally the body is cylindrical, 

 many-Jointed, the joints or segments much more distinct 

 than in the leech, and internally there are septa, or thin 

 muscular partitions, between them. The mouth is small, 

 forming an opening on the under side of the first segment. 

 On, or next to, the twenty-ninth to the thirty-sixth seg- 

 ments in Liimhricus terrestris is a flesh-colored swollen 

 portion called the cingulum or clitellum. 



The earth-worm is able to climb perpendicularly up boards, 



* Professor His admits that the bird passes through a stage compar- 

 able with the gastrula of other animals. (Whitman, p. 94.) 



