The Craspedomonadidae (often called Choanoflagellates, Fig. 40) are a group whose true nature was elucidated some forty years ago by the American zoologist, H. James-Clark. They are attached either to a substratum, by a stalk produced by the base of the cell, or to other members of the same colony; they are distinguished by the protrusion of the cytoplasm around the base of the single flagellum into a pellucid funnel,[135] in which the plasma is in constant motion, though the funnel retains its shape and size, except when, as sometimes happens, it is retracted. {122}[Illustration: FIG. 40.—Various forms of Craspedomonadidae. 2, _a_, Adult cell; 2, _b_, longitudinal fission; 2, _c_, the production of flagellulae by brood-formation; _c_, collar; _c.vac_, contractile vacuole; _fl_, flagellum; _l_, theca; _nu_, nucleus; _s_, stalk. (After Saville Kent.)] The agitation of the flagellum determines a stream of water upwards along the outer walls of the funnel; and the food-particles brought along adhere to the outside of the funnel, and are carried by its streaming movement to the basal constriction, where they are swallowed by the plasma, which appears to form a swallowing vacuole at that point. Longitudinal fission is the ordinary mode of reproduction, extending up through the funnel. If the two so formed continue to produce a stalk, the result is the formation of a tree-like stem, whose twigs bear at the ends the funnelled cells, or "collar-cells" as they are usually called. In _Salpingoeca_, as in so many other Flagellates, each cell forms a cup or theca, often of most graceful vase-like outline, the rim being elegantly turned back. _Proterospongia_ (Fig. 75, p. 182) secretes a gelatinous investment for the colony, which is attached to solid bodies. In this species, according to Saville Kent, the central members of the colony retract their collar, lose their flagellum, become amoeboid, and finally undergo brood-formation to produce minute zoospores. This is the form which by its differentiation recalls the Sponges, and has been regarded as a {123}transition towards them; for the flagellate, nutritive cells of the Sponges are provided with a collar, which exists in no other group of Metazoa (see pp. 171, 181, and Fig. 70, p. 176). The most recent monographer of the family is Raoul Francé, but James-Clark and Saville Kent did the pioneering work. [Illustration: FIG. 41.—_Opalina ranarum._ A, living specimen; B, stained specimen showing nuclei; C, stages in nuclear division; D-F, stages in fission; G, final product of fission; H, encysted form; I, young form liberated from cyst; K, the same after multiplication of the nucleus has begun. _nu_, Nucleus. (From Parker's _Biology_, after Saville Kent and Zeller.)]