132 FRESH- WATER MUSSEL -COMMON MARINE MUSSEL 



adductor muscle a short distance. At the posterior end of the mussel the lobes 

 of the mantle form the exhalent and inhalent siphons above described ; and 

 these are the openings of the two chambers into which -the mantle-cavity is 

 divided by a partition at the bases of the gills. 



3. Raise the mantle of the left side so as to expose the gills. Note that the 

 gills are long and plate-like (LameUibranchiata), and that there is a pair (an outer 

 and an inner giU) on each side between the median "foot " and the mantle. 



A 



Branchial filaments 



Blood vessel 



■ Outer lamella 

 Inter- lamellar junction 

 Inner lamella 



Internal ostium 



External ostium 



Blood vessel 



Fig. 44.^The Fresh-water Mussel {Anochnta). After Parker and Haswell. 



A. Transverse Section of an outer gill ; B. Transverse Section of an inner gill. 



Each gill consists of two lamellae, continuous along their anterior, ventral and 

 posterior borders. The gill has thus the form of a long and extremely narrow 

 bag open above. The cavity of the gill is subdivided by vertical strands of 

 tissue, the interlamellar junctions, extending between the lamellae and dividing 

 the cavity into a series of water-tubes, which are closed ventrally but open along 

 the dorsal edge of the gill. Each gill has a delicate striation, both horizontally 

 and vertically. , The vertical striation is due to the lamellae being composed of a 

 number of fine V-shaped gill-filaments, while the horizontal striation is due to 

 the interfilamentar junctions, horizontal strands of tissue connecting the fila- 

 ments. Between the filaments are minute apertures, the ostia, leading from 

 the mantle-cavity into the water-tubes. The gill-filaments are richly ciliated, 

 and the whole lamella is traversed by blood-vessels. [In Mytilus the gill- 

 filaments are loosely connected by inteflocking groups of cilia, the ciliary 

 brushes. Therefore the junctions are ciliary junctions, not solid interfilamentar 

 junctions.] 



