42 2 HOLMES. [Vol. XVI. 



similar group of cells is seen in the middle portion of the foot 

 in Crepidula (Conklin, '97, p. 143). Both Lillie and Conklin 

 derive the foot from cells of the second quartette. It seems 

 probable that, in Planorbis, cells from the third quartette also 

 enter into its formation. The cells immediately behind the 

 blastopore are derived from the third quartette, and it is not 

 unlikely that the median portion of the anterior end of the foot 

 is derived from some of these cells. 



The Larval Kidney. 



The larval kidney in Planorbis has the form of a F^shaped 

 tube, situated on either side of the body behind the head. At 

 the junction of the two arms is a large perforated cell, the 

 so-called giant cell, the lumen of which communicates with the 

 canals of the arms. The inner arm is directed toward the head; 

 it is formed of a row of perforated cells, and the lumen is cili- 

 ated and provided with a ciliated opening near the end. The 

 outer arm is directed downward and is provided with an opening 

 to the exterior. The movements of the cilia lining the arm of 

 the larval kidney may easily be seen in the living embryo. 



The larval or head kidney of the pulmonates has been observed 

 several times by the older writers on the embryology of these 

 forms (Stiebel, Ganin, Gegenbaur, Stepanoff), and it has been 

 mistaken for the rudiment of the nervous system, and also for 

 the oesophagus. The giant cell discovered by Biitschli rather 

 strangely had escaped the notice of Fol, who has given an 

 otherwise quite accurate description of this organ. 



The development of the larval kidney of pulmonates has been 

 studied by Fol, Rabl, and Wolfson, all of whom came to very 

 different conclusions regarding its origin. According to Fol, 

 the larval kidney in Planorbis makes its first appearance as 

 a small ectodermal pouch. " La poche s'approfondit dans la 

 direction du dos, de telle fagon qu'elle finirait par rencontrer 

 celle du cote oppos6 sur la ligne dorsale. Mais la croissance 

 dans ce sens s'arrete de bonne heure, I'organe se recourbe a 

 peu pr^s a angle droit et s'allonge maintenant dans la direction 

 de la bouche." The larval kidney, therefore, according to Fol, 



