It has been shown above that in the Actinotrocha larva there is one complete septum separating a closed posterior trunk coelom from an anterior pre-septal hasmocosl, which extends into the tentacles and into the pre-oral hood; that in front of the septum there is developed a second coelomic cavity, in the form of a horseshoe-shaped canal, extending into the base of each tentacle; that the nephridia open to the exterior ventrally behind the septum, and project freely into the pre-septal blood space, where they end blindly; that this inner extremity of the nephridium is furnished with a bunch of typical solenocytes resembling those of Polychastes and of Amphioxus; that during metamorphosis the pre-septal coelomic canal becomes the pre-septal adult cœlom, and that the pre-septal hæmocœl becomes reduced to the adult ring blood-vessel; that the nephridia lose their solenocytes and their connection with the ring vessel, and acquire new openings into the cœlomic cavities, probably by means of newly-formed coelomostomes or peritoneal funnels; that the longi-tudinal blood-vessels open into the pre-septal hæmocœl; and that a perianal sinus may be developed near the anal ciliated band. Much remains to be elucidated in the history of the nephridia; and the origin of the cavities has not yet been satisfactorily worked out. The possibility of the existence at a very early stage of a separate pre-oral coelom, coming into secondary continuity with the pre-septal hæmocœl through the breaking down of an originally complete anterior septum, has not been perhaps entirely excluded; but, so far as I am aware, there is no evidence in favour of the view that such is the real history of the cavity of the hood. The existence in Actinotrocha of a closed nephridium provided with solenocytes is an interesting and important fact, and its recognition enables us to add the Phoronidea to the already long list of animals (Vertebrata, Annelida, Echiuroidea, Mollusca, Endoproctous Polyzoa, Nemertina, Rotifera, and Platyhelmia) whose common ancestor must have possessed true nephridia of this nature. We may now expect to hear of the discovery of flame-cells or of solenocytes in the Sipunculoidea and Ectoprocta, which are doubtless related to Phoronis.