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(c.) The cavity of the tentacle is lined by a chitinous 

 sheath {/). 



(d.) Within this sheath the cavity is irregularly divided 

 by branching processes of connective tissue, within which 

 a granular white blood-corpuscle may occasionally be 

 found. 



2. Make a drawing showing these points. 



3. Examine a section which has passed through the 

 inter-tentacular, but not through the inter-lamellar junc- 

 tions (Fig. 152, a, a). 



Fig. 152. — Transverse section of 

 four gill-tentacles of Mytilus, as seen 

 in a transverse section of the two la- 

 mellae of a gill-plate. The section cuts 

 two tentacles of one lamella (the upper 

 in the figure) through the area of the 

 tentacular junctions; the lower tentacles 

 are cut between the tentacular jimctions. 

 (From " The Minute Structure of the 

 Gills of Lamellibranch Mollusca," by 

 R. Holman Peck. Quar. Jour. Mic. 

 Science, LXV., Jan, 1875.) 



a, a. Sections through the inter-ten- 

 tacular junctions of two tentacles of the 

 outer lamella, a', a. Sections of two 

 tentacles of the inner gill, between the 

 inter-tentacular junctions. b, h. The 

 bent cilia of the inter-tentacular junc- 

 tions, c. Space between the tentacles. 

 d. Tufts of cilia upon the outer edges of the tentacles, e, e, e, e. Cavities 

 of the tentacles. /, /. Chitinous lining of this cavity, g. Blood- 

 corpuscles within this cavity. 



(i.) Notice the cavity, the chitinous sheath, the exter- 

 nal epithelium, and the tufts of cilia, as in the preceding 

 section. 



(ii.) Notice also two pads (S, b) upon the sides of 

 the tentacle, formed by the thickening of the epithelium, 

 and carrying large hooked cilia. 



Fig. 152. 



