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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



attached by their proximal ends to the body wall. Myti- 

 lus differs from the Cyclas embryo in having the tentacles 

 bent upon themselves, so that their distal and proximal 

 halves are parallel, and side by side, and the two extremi- 

 ties near each other. Mytilus also differs from Cyclas in 

 having the distal ends of the tentacles united to each other, 

 as well as by the union of the 

 halves of the tentacle, through 

 inter-lamellar junctions, and also 

 by the slight adherence of adja- 

 cent tentacles by the inter-tentac- 

 ular junctions. 



a 



d 



a, a 



a 



FIG. 151. 



FIG. 151. Surface view of four gill- 

 tentacles of Mytilus edulis, magnified one 

 hundred and fifty diameters. (Drawn from 

 nature by W. K. Brooks. ) 



a, a, a, a. Gill tentacles. b, b, 6, b. Inter- 

 tentacular junctions, c, c, c. Inter-tentac- 

 ular spaces. 7, d, d, d. Cavities of ten- 

 tacles. 



c. Cut out a small piece of the unbroken gill of Mytilus, 

 and mount it in glycerine or balsam, in order to examine 

 its surface with a low power ; note : 



1. The gill tentacles, running side by side from the 

 dorsal margin to the ventral. 



2. A series of lines at right angles to the tentacles, and 

 much farther apart, the lines of inter-tentacular junction. 



3. With a higher power, notice the cavities of the ten- 

 tacles (Fig. 151, a, a, a, a), and the inter-tentacular 

 spaces (c, c, c). 



4. Notice that the wall of the tentacle becomes thick- 

 ened at intervals (b, 6, 6, >), thus giving rise to project- 

 ing pads upon the sides of the tentacle. 



