7. 



mantle with the base of the siphon, which line, as has beeii sho?m., 

 seems Tnorphologically to be a portion of the mantle's margin. 



During the development of the siphons the tentacle is 

 carried ventrally and in the adult, comes to lie nearly opposite 

 the ventral border of the inhalent siphon. Ihis unpaired tenta- 

 cle was first described by Brookp(l) and has frequently been re- 

 ferred to since. Pelseneer^ ■'■'*) fo-und that it occurred on either 

 the right or left side of Yoldia isonota, an obser\ration that al- 

 so h.olds true for Y. limatula. From its position and iner^ration 

 Pelseneer(l^) was led to conclude that it might be cojTpared to 

 the osphradium of gastropods, but this conclusion does not seem 

 to be borne out by a further knowledge of the subject. Another 

 organ corresponding in enervation and position to the osphradium 

 of other lamellibranchs, is present and will be described later. 



Ifflhen extended, figures 1, 3 and 10, st . the siphonal 

 tentacle is a long and slender filament, gradually tapering to 

 its free extremity, and generally lying loosely coiled on the bot- 

 tom of the aquariiim, or on the surface of the mud in -v^-ich the 

 animal lives. [Throughout its length it is set with small conic- 

 al, papillae, figures 6 and 7, sucin as have already been describ- 

 ed for the siphons. Each papilla, like the papillae on the 



