The Oligochaeta form a well-marked branch of that exceedingly large assortment of animals vaguely spoken of as worms, and embracing a number of types many of which have no near relationship to each other. From this great and unnatural group, which has survived as "Vermes" even in some quite modern text-books, we can separate off those forms which show a plain segmentation or division of the body internally as well as externally into a series of more or less similar rings, as a Class Chaetopoda. This Class, consisting of the Orders mentioned on p. 241, includes the worms which form the subject of the present chapter—the Oligochaeta, as they were originally called by Grube, on account of the fewness of their chaetae as compared with the number possessed by the majority of the Polychaeta. Our knowledge of this group, as of so many others, dates from Aristotle, who called the earthworms the "intestines of the earth." But it is only very recently that the numerous and remarkable genera of exotic earthworms have been anatomically investigated; indeed the common British species was not really well known before the publication of the memoirs of Lankester[401] and Claparède[402] in 1864 and 1868, in spite of the elaborate quarto devoted to it by Morren,[403] the botanist, in 1826. {348}Some of the aquatic species afforded material to Bonnet and Spallanzani for their experiments upon the powers of regeneration of the animals when cut into fragments, while the work of O. F. Müller[404] upon various Naids is a monument of careful anatomical description. Our knowledge of the aquatic Oligochaeta does not appear to have advanced so rapidly as has that of the earthworms.