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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



(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- 

 mellfe 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 junctions. 

 (From " The Minute Structure of the 

 Gills of Lamellibranch Mollusca," by 

 R. llolman 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, b. 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 tw T o pads (b, b) upon the sides of 

 the tentacle, formed by the thickening of the epithelium, 

 and carrying large hooked cilia. 



FIG. 152. 



