4 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



they are frequently rendered of equal length by injury. Both siphons, 

 Fig. 14, have, between their outer and inner layers of epithelium, large 

 bundles of longitudinal muscle fibres, separated by sheets of radial 

 muscles. A few circular fibres lie near the epithelium, but they are not 

 numerous. The siphons are extended by forcing blood into the spaces 

 hollowed out in the connective tissue. If the siphon of a young specimen 

 is examined with a moderately high power of the microscope, small, 

 conical papillae will be seen projecting from its surface, Fig. 15. Each 

 papilla bears at its tip a long, rigid filament. Older specimens show 

 similar papilla^ but instead of bearing a single elongated filament, 

 several shorter filaments are borne on the tip of each. Similar papilla? 

 are found in various places on the mantle and its modifications. They 

 probably correspond to the " Pinselzellen " of Fleming, 3 later described 

 by Dorst' 2 and Rawitz. 13 



Siphonal tentacle and marginal tentacles. The youngest specimen 

 collected with a dredge shows a little rounded knob, Fig. 11, st., lying 

 on one side, between the mantle and the siphon. This knob, the 

 rudiment of the siphonal tentacle, is an outgrowth from the line of 

 union of the mantle with the base of the siphon, which line, as has 

 been shown, seems morphologically to be a portion of the margin of 

 the mantle. 



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. This unpaired tentacle was first 

 described by Brooks 1 and has frequently been referred to since. 

 Pelseneer 10 found that it occurred on either the right or left side of 

 Yoldia isonota, a'n observation that also holds true for Y. limatula. 

 From its position and innervation Pelseneer 10 was led to conclude that 

 it might be compared to the osphradium of gastropods, a conclusion 

 that he later modified. 11 Another organ corresponding in innervation 

 and position to the osphradium of other lamellibranchs is present 

 and will be described later. 



When extended, Figs. 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 bottom of the aquarium, or on 

 the surface of the mud in which the animal lives. Throughout its 

 length it is set with small, conical papilhe, Figs. 6 and 7, such as have 

 already been described for the siphons. Each papilla, like the papillse 

 on the adult siphon, bears a cluster of filaments at its tip. 



