_Larva._--The larva is a typical trochosphere, which, although of a temporary character, shows a distinct segmentation of the mesoblast, of the nervous system, and of the ciliated and pigmented structures in the skin, resembling that of Chaetopods. The preoral lobe persists as the proboscis. The sexes of the larvae are not determinable in the early stages, but when a certain growth has been reached in Bonellia the males seek the proboscis of the adult females, and passing into the mouth undergo there the transformation into the planarian-like parasite which is the fully-formed male. This now creeps along the body of the female and takes up its home in her nephridia. _Classification and Distribution._--The Echiuroidea consists of the following genera:--(1) _Bonellia_ (Rol.), with four species, widely distributed, but inhabiting the temperate and warmer waters of each hemisphere. (2) _Echiurus_ (Guérin-Méneville), with four species. This genus reaches from the Arctic waters of both hemispheres into the cooler temperate regions. (3) _Hamingia_ (Kor. and Dan.), with one species, which has been taken in the Arctic Sea and the Hardanger Fjord. (4) _Saccosoma_ (Kor. and Dan.) was described from a single specimen dredged about half-way between Iceland and Norway. (5) _Thalassema_ (Gaertner, Lamarck), with twenty-one species. This genus is in the main a denizen of the warmer waters of the globe. Sixteen species are found only in tropical or subtropical seas, three species are Mediterranean (_Mt. Stat. Neapel_, 1899), whilst three species are from the eastern Atlantic, where the temperature is modified by the Gulf Stream (Shipley; see Willey's _Zoological Results_, part iii. 1899; _Proc. Zool. Soc. Lond._, 1898, 1899; and _Cambridge Natural History_, ii.). The following are found in the British area:--_E. pallasii_ (Guérin-Méneville), _Th. neptuni_ (Gaertner), and _Th. lankesteri_ (Herdman, _Q.J.M.S._, 1898). _Affinities._--The occurrence of trochosphere larva and the temporary segmentation of the body have led to the belief that the Echiuroids are more nearly allied to the Annelids than to any other phylum. This view is strengthened by certain anatomical and histological resemblances to the genus _Sternaspis_, which in one species, _S. spinosa_, is said to carry a bifid proboscis resembling that of the Echiuroids. (A. E. S.)