STRUCTUKE OF THE ULMELLIBRANCHIATE GIUL. 305 



(iii.) Notice that the hooks of adjacent tentacles inter- 

 lock to form the inter-tentiicular junctions. 



(iv.) Draw the section. 



4. Examine a section which has cut the inter-lamellar 

 junctions. (Fig. 153.) 



(i.) Notice that the inner ends of the outer and inner 

 halves of each tentacle are united (Fig. 153, A), and the 

 canities (e, e) of the two sides are continuous across 

 the neck (?), thus formed. 



(ii.) The chitinous linings of 

 the two divisions of the tentacle 

 line only the outer ends of this 

 cavity {f), and do not extend 

 into the central portion. 



(iii.) Draw the section. 



Fig. 153. — Transverse section of four 

 gill-tentacles of Mj-tilus. through the inter- 

 tentacular and inter-lamellar junctions. 

 (From Peck.) 



A, B, C, B, E, F, and G. as before. 

 U. Inter-lamellar jtmction. I. Cavity 

 of the inter-lamellar jimction, continuous 

 with the tentacular cavity E. 



Fig. 153. 



III. The Gin of Unio. 



Remove the gills from one side of the body by cutting 

 their attachments to the mantle and body ; place them in 

 water for examination. Each of the four gills is now seen 



be a flat plate, with a nearly straight dorsal margin by 

 rhich it is attached to the body, and a slightly curved 

 rentral margin, which is free. 



a. Examine the dorsal mararin of one of the s^Us, and 

 lote that it is made up of two parallel plates, the two 

 lamellfe, which are united at intervals by cross partitions, 

 le inter-lamellar junctions. 



