STRUCTURE OF THE LAMELLIBRANCHIATE GILL. 301 



other, but in the opposite direction, and the proximal 

 halves of the tentacles here form the outer lamella, and 

 the distal halves the inner lamella. 



4. The points of attachment of the gill tentacles to each 

 other. 



(i.) All the tentacles of a gill are attached to each 

 other, and to the body along the line i. 



(ii.) The distal ends of the tentacles are united to 

 form the ridge (&) which forms the dorsal margin of the 

 outer lamella of the inner gill, and of the inner lamella of 

 the outer gill. 



(iii.) Each tentacle is very slightly united to the ad- 

 jacent tentacles by junctions which give Avay to the slight- 

 est strain, and which are represented diagrammatically 

 by the dots upon the right half of Fig. 150. These 

 points of union may be termed the inter-tentacular junc- 

 tions. 



(iv.) Upon attempting to straighten a tentacle, the two 

 halves will be found to be fastened together by bands 

 which run from the inner to the outer lamella. These 

 bands, which may be called the inter-lamellar junctions 

 (Fig. 150, e), are formed by the meeting and fusion of 

 the walls of the two halves of the tentacle, which cannot 

 be separated without rupturing the connecting band. 



( v. ) Each tentacle is hollow, and its cavity (Fig. 150,/"), 

 communicates with the longitudinal blood-vessels (&). At 

 the points of inter-lamellar junction, the cavity of the 

 descending portion of the tentacle communicates with that 

 of the ascending portion, as shown in the left side of 

 Fig. '150. 



The resemblance between the embryonic gill of Cyclas 

 and that of the adult Mytilus will be readily perceived. 

 In each the gill is made up of a row of parallel tentacles, 



