Chap. VI.] 



GILLS OF MOLLUSC A. 



219 



developed. The mechanism of these gills is simple 

 enough ; projecting into the water they are moved 

 about in it, and minor currents are produced around 

 them by the action of the cilia which cover their 

 surface. 



A very simple type of gill is found in some of the 

 Liamellibraiichiata, where the Cteiiidium (see 

 page 79) consists essentially of two rows of hollow 

 filamentar processes of the body wall extended along 

 either side of the foot, and ciliated externally ; these 

 are the gill filaments ; each filament, at its free 

 end, bends upwards in such a way as to leave a space 

 between the ascending and descending portions ; the 

 free end of the ascending portion of the outer filament 

 may either be free, as in the sea-mussel (Mytilus), or 

 become at- 

 tached to 

 the mantle 

 as in the 

 fresh - water 

 mussel (An- 

 odon) ; it is 

 plain, then, 

 that its as- 

 cending lies 

 externally 

 to its de- 

 seen ding 

 portion ; the 



inner gill filament has its ascending portion internal to 

 the descending, and so it results that in transverse sec- 

 tion the two gill filaments have the form of a W, the 

 median upper point of which represents the point 

 where the two filaments are inserted (or arise from) 

 the body wall. The ciliated epithelial cells are often 

 particularly well developed at certain spots ; the cilia 

 of these " ciliated junctions " interlock with those 



n/ 



Fig. 94. Transverse Section of Arenicola. 



D. Dorsal ; v, ventral side ; n, ganglionic chain ; /, intes- 

 tine ; br, gills; v, ventral vessel; d, dorsal; v', visceral 

 vessel ; p, vessel around intestine ; a, &, vessels of gills. 

 (.After Gegenliaur.) 



