GENERAL AXAT03IY OF A LAMELLIBBA^CH. 277 



notice that the bristle has passed into a chamber which is 

 dorsal to the gills, and which is known as the cloacal 

 chamber. 



j. The gills. Notice that the upper edge of each gill 

 carries a row of openings which communicate with the 

 cloacal chamber. These are the openings of the vertical 

 water tubes. Pass a l)ristle into one of tlie water tubes, 

 and notice that this ends blindly at the free ventral edge 

 of the gill. Xotice also that it is separated bv vertical 

 partitions from the water tubes before and behind it. 

 "VYlien the. microscopic structure of the gill is studied as 

 described in Section XXVI 1. each water tube will be 

 seen to communicate with the branchial chamber throuofh 

 a 2rcat numljer of microscopic ciliated openings, the bran- 

 c]i.i"^ si Us, Avhich cover the flat surfaces of the gill. The 

 water which is drawn through tlie l)ranchial syphon into 

 the l>ranchial chamber is driven by the cilia through the 

 branchial slits into the water tu'x'S. and as these are filled 

 the water flows up into the cloacal chanilier. and is dis- 

 charged from the body through the cloacal siphon. 



1. Each of the four gills consists of two flat plates, the 

 outer and inner lamellae (Fig. 143, e.r and in ) , and these are 

 united to each other by vertical })artitions, Avhich separate 

 tlie water tubes from each other. 



2. The up|ier edge of the outer lamella of each outer 

 gill is united to the mantle. 



3 . The upper edge of the inner lamella of the outer gill 

 is united to that of the outer lamella of the inner gill, and 

 the anterior third is also united to the wall of the abdomen. 



4. The inner lamella^ (e) of the inner gills are united to 

 each other for about one third of their length at the pos- 

 terior end of the body, but at the posterior end of the 

 abdomen they se})arate and pass one on each side of it. 



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