68 



EDWARD L. RICE. 



(Figs. 4, C '; 4, i7), and may be followed readily into the charac- 

 teristic filaments of their respective gills (Figs. 2 and 3). 



In this connection it should be strongly emphasized that the 

 filament anlagen here described are by no means the equivalent of 



the papillae described by Lacaze- 

 Duthiers, although their appear- 

 ance in such a stage as that figured 

 in Fig. 4, D, is strikingly similar. 

 The undivided transverse fold (Fig. 

 4, A) contains potentially an entire 

 filament of each gill ; and each of 

 its subdivisions (Fig. 4, D) is the 

 equivalent of the two limbs of a 

 filament placed side by side. On 

 the other hand, the earlier type of 

 anlage must be considered as the 

 equivalent of only one limb of the 

 definitive filament, or perhaps better, 

 of the two placed end to end. In 

 the earlier type the reflexed limb of 

 the filament originates through a 

 bending of the anlage ; in the later 

 type there is no bending, but rather 

 a longitudinal splitting. The exact details of this splitting and 

 the formation of the cavity within the gill, shown in Fig. 4, E, 

 have not been worked out ; but a study of a series of specimens 

 such as those represented in Fig. 4 makes it clear that the process 

 consists essentially in a thinning and ultimate perforation of the 

 plate-like anlage of Fig. 4, D. 



Corroborative evidence as to the nature of these later filament 

 anlagen and their distinction from the earlier and more anterior 

 ones is found in the character of the ciliation. A comparison of 

 Figs. 4 and 5 is instructive in this connection. Fig. 5 is drawn 

 from a section ; but the filaments of the gills have been slightly 

 reconstructed from neighboring sections, so that they are repre- 

 sented in their entirety. This represents an earlier filament of 

 the outer gill (Fig. 5, b), with its characteristic rod-like form. 

 Details of the ciliation have been omitted except for the some- 



Fig. 3. Posterior tip of ctenid- 

 ium of Mytilus of 4.5 mm. length, 

 viewed from inner side. Magnifica- 

 tion 170. 



