EMBRYOLOGY OF LEECHES. 167 
The early phases in the embryological development of the 
leech (Clepsine) 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 
the 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 Clepsine. 
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-layered 
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 (Lumbricus terrestris Linn., Fig. 115) 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 Lumbricus 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.) 
