igoS] EXCRETORY ORGANS OF METAZOA. 581 



more numerous and the inner end is composed of a number of large 

 amoeboid cells all of which have long cilia; for a while the inner end 

 may be open (as described by Rabl, 1879, and Erlanger, 1894) since 

 the cells there may become loosened from their epithelial connection, 

 but later this end becomes completely closed even though at places 

 by a very thin membrane. These are the most complicated larval 

 nephridia found in Gasteropods ; they subsequently degenerate com- 

 pletely. As to the development of these head kidneys: Rabl (1879) 

 and Holmes (1900) considered them mesoblastic; Erlanger (1893) 

 interpreted them as mainly mesoblastic with a portion of the duct 

 ectoblastic, and Pelseneer (1901) stated that the large distal portion 

 is ectoblastic. But Fol (1880), Wolfson (1880) and Aleisenheimer 

 (1898) concluded that they are entirely ectoblastic; the last named 

 investigator speaks of them as arising as paired tubular invaginations 

 at the level of the proctodseum. 



(d) Pelecypods {LamelUhranchs). — In Teredo there is a pair 

 of ciliated Urnieren in the young larva (Hatschek, 1880). Only 

 the left one is developed in Cyclas, and opens externally in the 

 region of the head vesicle; it consists of three highly complex cells 

 with intracellular cavity, the innermost branched cell closing it 

 from the hlastocoel (Stauffacher, 1898). In Dreissensia each of the 

 larval kidneys consists of three cells, the innermost provided with 

 a ciliary flame and closing the canal, the next forming the tube, and 

 the third constituting a duct connecting with the surface (Meisen- 

 heimer, 1901a). With regard to the embryogeny, Hatschek de- 

 scribed these organs as appearing first at the anterior ends of the 

 mesoblastic bands, at first with no connection v/ith the ectoblast, 

 and concluded that the nephridium of each side " is probably derived 

 from only one or a few mesoderm cells " ; Staufifacher held that in 

 Cyclas only the innermost cells is mesoblastic and the others ecto- 

 blastic; while Meisenheimer (Dreissensia, 1901a, Cyclas, 1901&) 

 described them as arising conjointly with the heart and pericardium 

 from the ectoblast. 



Homologies of the Larval Nephridia. — Salensky (1872) and 

 Bobretzky (1877) homologized the outer kidneys of prosobranchs 

 with the Urnieren of Pulmonates. Biitschli (1877) suggested that 

 the Urnieren of Pahidina are possibly homologous with those of 



