MOLLUSC A. 233 



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 Lymnaiiis, Planorbis, etc. It consists of a paired 

 V-shaped ciliated tube with a pedal and cephalic limb. The former 

 is certainly without 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 Pulmouata (Lyrumeus, 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 Lyinnajus 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. 188) 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. 



Permanent 'renal organ. According to the most recent observer 

 (Rabl, No. 268), whose statements are supported by the sections figured, 

 the permanent renal organ in Gasteropods is developed from a mass 

 of mesoblast cells close to the end of the intestine. This is first 

 carried somewhat to the left side, and then becomes elongated and 

 hollow, and attaches itself to the epiblast on the left side of the anus 

 (fig. ]08, r). After the formation of the heart the inner end opens 

 into the pericardium and becomes ciliated, the median part becomes 

 glandular and concrements appear in its lining cells, and the terminal 

 part forms the duct. 



Previous observers have usually derived this organ from the epiblast ; 

 according to Rabl this is owing to their having studied too late a 

 stage in the development. 



In Cephalopoda the excretory sacks or organ of Bojanus are 

 apparently differentiations of the mesoblast \ At an early stage part 

 of tbeir walls envelopes the branchial veins. From this part of the wall 



1 I conclude this from Bobretzky's figures. 



