BEANCHLE OF MOLLUSCA. 337 



filaments grow together this space traverses the whole of the gill- 

 plate, and communicates with the exterior by means of the clefts 

 between the filaments. The water which enters by these clefts is 

 collected into a canal at the point where the plate is attached ; and 

 is carried by it to the hinder end of the body. 



There are chitinous rods in each of the gill-filaments, which form 

 a special organ of support. 



The surface of the whole gill is covered by ciliated epithelium. 

 Eows of large cilia extend along the ridge-like projections of the 

 gills; between these there are finer closely-packed cilia, and the 

 two together keep up a continual stream of water. There is a 

 groove at the free edge of each gill-plate ; this is formed by depres- 

 sions on each of the gill-lamellas and is invested by longer cilia ; it 

 leads to the mouth, and so produces a current of waterj which is 

 well adapted to bring in nutriment. 



Great modifications of this system are brought about by the 

 fusion of the gills of the two sides; this, which obtains posteriorly 

 to the fqot, is either effected by the direct junction of the free edges, 

 or by the development of a special membrane, which unites the 

 gills of either side together. This fusion is best seen in the falci- 

 form curved gill -plates of Anomia, where the whole branchial 

 apparatus is separated from the greatly-reduced visceral sac, and is 

 no longer lateral in position. 



[R. noLMAN Peck, Tho structure of the Lamellibranch gill. Quart. Journal 

 Microsc. Sci. 1876.] 



Bonnet, R., Der Bau u. die Circulationsverhaltnisse dcr Acephalenkieme. 

 Morphol. Jahrb. III. 



§ 261. 



The branchial apparatus of the Gastropoda, though greatly 

 varied in details, is arranged in very much the same way as in tho 

 Lamellibrauchiata ; that is to say, it is typically made up by lamellfc, 

 or by more distinctly cylindrical processes, which are arranged 

 parallel to one another. These project from the surface of the body, 

 and are, therefore, bathed by the surrounding medium, the water, 

 while a current of blood passes along them internally. This simi- 

 larity is still more marked by their position relatively to the mantle, 

 for they have just the same relation to it as have the gills of the 

 Lamellibrauchiata. As compared with these latter they are less 

 numerous and more confined ; their structure is comparatively much 

 simpler. The epipodial gill is arranged circularly in the Placophora, 

 as it is also in Patella ; but in other Patellidse (Lottia) the two pinnate 

 branchiee are more distinctly dorsal, so that they seem to be different 

 from the epipodial gills. Fissurella and Emarginula also have their 

 two gills placed in the anterior region and below the mantle. In 

 Haliotis also, they are distinctly paired, but they are placed more to 

 the left hand of the animal. They are also characteristically modified 

 in the Zeugobranchia. In the Ajiisobranchia the left gill is smaller, 

 and the right one more largely developed ; this arrangement, which is 



