1904.] NATURAL SCIENCES OF PHILADELPHIA. 379 



minutely described a group of ectodermal cells lying laterally just 

 behind the velum and probably arising from the second quartet; they 

 become much vacuolated, filled with darkly stained granules and be- 

 fore metamorphosis separate from the ectoderm and are lost. Erlanger 

 (1892) concluded that the larval kidney of Bythinia was partly ecto- 

 dermal and partly mesodermal, and had no connection with the 

 definitive kidney of the adult. The earlier results of Blitschli (1877) on 

 Paludina as well as Bythinia were enlarged by Erlanger (1891-2), 

 showing that in these fresh-water Prosobranchs the larval kidney 

 was formed from inner mesodermal and outer ectodermal por- 

 tions. 



Rabl (1879) established a mesodermal origin for the primitive kidney 

 of Planorbis, and Holmes (1900) in his late work confirms the same. 

 Fol (1879) derived the larval kidney of Planorbis entirely from the 

 ectoderm. Wolfson (1880) described the larval kidney of Limnaa 

 as arising from a large velar cell on either side which migrates inward, 

 retaining connection with the exterior through an intra-cellular duct. 

 Meissenheimer (1898) says of Limax, we have "in der urniere ein rein 

 ekto-dermales Gebilde vor uns, zu dem das ^lesoderm auch nicht den 

 geringsten Beitrag geliefert hat." From his figures and discussion it 

 appears very evident that in this form the primitive kidney is purely 

 ectodermal in origin. In 1899 Meissenheimer published his investiga- 

 tions on the "Urniere der Pulmonaten" (of the Basommatophora, 

 Ancylus, Physa, Planorbis, Limncea, and of the Stylommatophora, 

 Succinea, Helix, Avion, Limax). In both these groups he shows the 

 larval kidney to be entirely ectodermal in origin and similar in struc- 

 ture, the urinary tube of the latter group being many-celled, while in 

 the former but four cells comprise it. In both a ciliated cell or cells 

 closes the inner end of the tube, and for this reason Meissenheimer com- 

 pares the primitive kidney of the Pulmonate with the end cells of the 

 water vascular system of the Platyhelminthes. 



Among the Lamellibranchs Hatschek (1880) describes the larval 

 kidney of Teredo as probably both ecto- and mesodermal in origin. 

 In the single left primitive kidney of Cyclas, Stauffacher (1897) found 

 a similar though more complicated structure arising from Ijoth ecto- 

 dermal and mesodermal elements. 



Meissenheimer (1901) finds that in Dreissensia polymorpha the larval 

 kidneys arise from ectodermal cells wholly, each of the two being formed 

 from a few in-wandering cells. ' The structure is more simple than that 

 of the Pulmonates and Meissenheimer suggests that it may be the 

 ground type of the group. This might then be described as an ecto- 



