Most Limnadiidae resemble the Cladocera in the possession of a “dorsal organ” quite distinct from the above; in _Limnetis_ and _Estheria_ it has the form of a small pit, lined by an apparently glandular ectoderm, and this is its condition in many Cladocera; in _Limnadia lenticularis_ it is a patch of glandular epithelium on a raised papilla. _Limnadia_ has been observed to anchor itself to foreign objects by pressing its dorsal organ against them, and many Cladocera do the same thing; _Sida crystallina_, for example, will remain for hours attached by its dorsal organ to a waterweed or to the side of an aquarium. Structures resembling a dorsal organ occur in the larvae of many other Crustacea, but the presence of this organ in the adult is confined to Branchiopods, and indeed in many Cladocera it disappears before maturity. It is certain that the sensory and adhesive types of dorsal organ are not homologous, especially as rudimentary sense-organs may exist on the head of Cladocera together with the adhesive organ. The =telson= differs considerably in the different genera. In the Branchipodidae[17] the anus opens directly backwards; and the telson carries two flattened backwardly directed plates, one on each side of the anus, the margins of each plate being fringed with plumose setae. In _Artemia_ the anal plates are rarely as large as in _Branchipus_, and never have their margins completely fringed with setae; in _A. salina_ from Western Europe, and in _A. fertilis_ (Fig. 4, A) from the Great Salt Lake of Utah, there is a variable number of setae round the apical half of each lobe, but in specimens of _A. salina_ from Western Siberia the number of setae may be very small, or they may be absent; in the closely allied _A. urmiana_ from Persia the anal lobes are well developed in the male, each lobe bearing a single terminal hair, but they are altogether absent in the female. Schmankewitch and Bateson have shown that there is a certain relation between the salinity of the water in which _Artemia salina_ occurs and the condition of the anal lobes, specimens from denser waters having on the whole fewer setae; the relation is, however, evidently very complex, and further evidence is wanted before any more definite statements can be made. [Illustration: FIG. 4.—=A=, Ventral view of the anal region in _Artemia fertilis_, from the Great Salt Lake; =B=, ventral view of the telson and neighbouring parts of _Lepidurus productus_; =C=, side view of the telson and left anal lobe of _Estheria_ (sp.?). ] In the Apodidae the anal lobes have the form of two-jointed cirri, often of considerable length; in _Apus_ the anus is terminal, but in _Lepidurus_ (Fig. 4, B) the dorsal part of the telson is prolonged backwards, so as to form a plate, on the ventral face of which the anus opens, much as in the Malacostraca.