Renal organs. Amongst the Gasteropods and Pteropods there are present provisional renal organs, which may be of two kinds, and a permanent renal organ. The provisional organs consist of either (1) an external paired mass of excretory cells or (2) an internal organ provided with a duct, which is not in all cases certainly known to open externally. The former structure is found especially in the marine Prosobranchiates (Nassa, etc.) where it has been fully studied by Bobretzky. It consists of a mass of cells on each side of the body, close to the base of the foot, and not far behind the velum. This mass grows very large, and below it may be seen a continuous layer of epiblast. The cells forming it fuse together, their nuclei disappear, and numerous vacuoles containing concretions arise in them. At a later stage all the vacuoles unite together and form a cavity filled with a brown granular mass. The provisional internal renal organ is found in many pulmonate Gasteropods--Lymnæus, Planorbis, etc. It consists of a paired V-shaped ciliated tube with a pedal and cephalic limb. The former has an external opening, but the termination of the latter is still in doubt. It consists, according to Büschli's description (No. 244), in the freshwater Pulmonata (Lymnæus, Planorbis) of a round sack, close to the head, opening by an elongated and richly ciliated tube in the neighbourhood of the eye. From the sack a second shorter tube passes off towards the foot, which seems however to end blindly. The cells lining the sack contain concretions, and there is one especially large cell in the lumen of the sack attached on the side turned towards the eye. It coexists in Lymnæus with provisional renal organs of the type of those in marine Prosobranchiata. A somewhat different description of the structure and development of this organ in Planorbis has recently been given by Rabl (No. 268). It consists of a V-shaped tube on each side with both extremities opening into the body cavity. The one limb is directed towards the velar area, the other towards the foot. It is developed from the mesoblast cells of the anterior part of the mesoblastic band. The large mesoblast (p. 227) of each side grows into two processes, the two limbs of the future organ. A lumen in the cell is continued into each limb, while continuations of the two limbs of the V are formed from the hollowing out of the central parts of the adjoining mesoblast cells. In Limax embryos Gegenbaur found a pair of elongated provisional branched renal sacks, the walls of which contained concretions. These sacks are provided with anteriorly directed ducts opening on the dorsal side of the mouth. This organ is probably of the same nature as the provisional renal organ in other Pulmonata.