MOLLUSCA. 28 1 



The provisional organs consist of either (i) 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 pulmo- 

 nate Gasteropods Lymnaeus, 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 Biitschli's description (No. 244), in the fresh- 

 water Pulmonata (Lymnasus, 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 Lymnasus 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 



