562 MOLLUSC A. 



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 in- 

 dependent 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 pos- 

 sibly 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. I. No. 268) and Hatschek hold that it is de- 

 veloped 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 the continua- 

 tions of the two limbs are formed by perforated mesoblast cells. 



According to Fol these organs originate in aquatic Pulmonata as 

 a pair of invaginations of the epiblast, slightly behind the mouth. 

 Each imagination grows in a dorsal direction, and after a time 

 suddenly bends on itself, and grows ventralwards and forwards. It 

 thus acquires its V-shaped form. 



In the terrestrial Pulmonata the provisional excretory organs are, 

 according to Fol, formed as epiblastic invaginations, in the same way 

 as those in the aquatic Pulmonata, but have the form of simple non- 

 ciliated sacks, without internal openings. 



The permanent renal organ of the Mollusca consists typically of a 

 pair of tubes, although in the majority of the Gasteropoda one of 

 the two tubes is not developed. It is placed considerably behind 

 the provisional renal organ. 



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

 Gasteropod larvae, ride Vol. i. p. 232. 



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

 et gener., Vol. vin. 



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

 lirnb having an external opening, and the statement to the reverse effect on p. 232 of 

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

 corrected. 



