published for the first time, I have thought it wiser to keep to the accepted relations in the general description, a course which has the advantage of avoiding difficulties in the study of the literature of the Class. The supposed resemblance of _Pedalion_ to the Crustacea is probably the result of convergence, not of consanguinity. The Polyzoa are a group of freely-budding organisms whose structure otherwise recalls in many respects that of the attached Rotifers; but a close investigation reveals so many differences in structure, orientation, and development, that we cannot regard the two groups as at all closely allied. Thus the Rotifers may be regarded as a group apart, but probably representing an early offshoot from a free-swimming Platyhelminth, probably a Rhabdocoele; the modifications being the loss of the general ciliation of the surface, the arching of the back into an elongated vault, the conversion of the inner half of the pharynx into a gizzard, the change of position of the genital and urinary apertures to the antero-dorsal surface, and the opening of the intestine into the genito-urinary cloaca. GASTROTRICHA. This small and very homogeneous group consists of minute fresh-water organisms, closely resembling many Ciliate Infusoria in their movements, habit and habitat. They were first described in detail by Ehrenberg, and placed by him and Dujardin in the neighbourhood of Rotifers. In recent years A. C. Stokes[287] {232}in America and C. Zelinka[288] in Germany have contributed, the former a careful description of a number of new species and their habits, the latter a complete monograph of everything that is known of the Order. The Gastrotricha dwell among filamentous Algae and organic débris, and are of frequent occurrence with Protozoa and Rotifera of similar habit. The largest known measures only 400 µ (1/60 in.) in length, and the smallest run as low as 74 µ (1/300 in.). [Illustration: FIG. 119.—Gastrotricha. (From Zelinka.) A, _Chaetonotus bogdanovii_, side view (after Schimkewitsch); B, _Gossea antenniger_ (after Gosse); C, _Dasydetes goniathrix_ (after Gosse); D, _Dasydetes saltitans_ (after Stokes); E, _D. longisetosum_ (after Metschnikoff); F, _Chaetonotus spinulosus_ (after Stokes); G, _Chaetonotus schultzei_ (after Gosse and Bütschli). (Magnified.) B-F, × about 390; G, × about 125.]