STRUCTURE OF THE LAMELLIBRANCHIATE GELL. 309 



in depth, some being quite shallow, while others (i^, F)^ 

 open into the water cavity. 



b. Make a sketch of the section. 



c. Examine a portion of the section with a higher 

 power, two hundred and fifty diameters, noticing : — 



1. The nearly oval cross sections of the external mar- 

 gins of the gill tentacles (Fig. 155, g, g^ g.) 



2. The narrow necks by which these are joined to the 

 body of the lamella (r). 



Fig. 155. 



Fig. 155. — Transverse section of a portion of the ^ of Unio pur- 

 purea, magnified two hundred and fifty diameters. (Drawn from nature 

 by W. K. Brooks.) 



a. Inter-lamellar water-tube. c. Outer lamella. /. Inhalent ostium. 

 g. Gill-tentacles, h. Their cilia, k. Their cavities. I. Chitinous rods. 

 0. Inter-tentacular furrows, p. Epithelial lining of water-tube. p'. Epi- 

 thehal linin» of inhalent ostium, r. Lamella. 



3. The cross sections of the channels (o, o, o) between 

 the tentacles. 



4. Some of these channels will be found to penetrate 

 the whole thickness of the lamella, as at f, thus opening 

 into the water tube (a). 



