EXCRETORY ORGANS. 68 1 



The two large tubes open to the exterior, either by means of 

 a median posteriorly placed contractile vesicle, or by a pair of 

 vesicles, which have a ventral and anterior position. The former 

 type is characteristic of the majority of the Trematoda, Cestoda, 

 and Rotifera, and the latter of the Nemertea and some Trematoda. 

 In the Turbellaria the position of the external openings of the 

 system is variable, and in a few Cestoda (Wagner) there are 

 lateral openings on each of the successive proglottides, in addition 

 to the terminal openings. The mode of development of these 

 organs is unfortunately not known. 



Mollusca. In the Mollusca there are usually present two 

 independent pairs of excretory organs one found in a certain 

 number of forms during early larval life only 1 , and the other 

 always present in the adult. 



The larval excretory organ has been found in the pulmonate 

 Gasteropoda (Gegenbaur, Fol 2 , Rabl), in Teredo (Hatschek), and 

 possibly also in Paludina. It is placed in the anterior region of 

 the body, and opens ventrally on each side, a short way behind 

 the velum. It is purely a larval organ, disappearing before the 

 close of the veliger stage. In the aquatic Pulmonata, where it is 

 best developed, it consists on each side of a V-shaped tube, with 

 a dorsally-placed apex, containing an enlargement of the lumen. 

 There is a ciliated cephalic limb, lined by cells with concretions, 

 and terminating by an internal opening near the eye, and a non- 

 ciliated pedal limb opening to the exterior 3 . 



Two irreconcilable views are held as to the development of 

 this system. Rabl (Vol. II. No. 268) and Hatschek hold that it 

 is developed in the mesoblast; and Rabl states that in Planorbis 

 it is formed from the anterior mesoblast cells of the mesoblastic 

 bands. A special mesoblast cell on each side elongates into two 

 processes, the commencing limbs of the future organ. A lumen 

 is developed in this cell, which is continued into each limb, while 



1 I leave out of consideration an external renal organ found in many marine 

 Gasteropod larvre, vide Vol. n. p. 280. 



2 H. Fol, "Etudes sur le devel. d. Mollusques. " Mem. in. Archiv d. Zool. 

 expcr. et gencr., Vol. vm. 



3 The careful observations of Fol seem to me nearly conclusive in favour of this 

 limb having an external opening, and the statement to the reverse effect on p. 280 of 

 Vol. n. of this treatise, made on the authority of Rabl and Butschli, must probably be 

 corrected. 



