540 ORGANS OF RESPIRATION. 



the viscera, the labial appendices, the mantle, and its fimbri- 

 ated tentacular prolongations, and even line the interior of the 

 alimentary canal. A pair of double pectinated branchial folds, 

 on each side of the foot, within the mantle, are suspended 

 in the branchial cavity, fixed by their two contiguous upper 

 margins, free below, and at their two upper remote margins, 

 each fold forming a loose, compressed, reticulate sac, and 

 supporting on its innumerable minute elongated meshes the 

 capillaries of the branchial blood vessels. In the conchifera, 

 which burrow in rocks, timber, mud, sand, or other materials, 

 the mantle and the respiratory cavity are necessarily pro- 

 longed, to bring the two orifices of the palleal cavity within 

 reach of the surrounding element, and the branchiae are 

 greatly elongated. 



Each pendent gill is composed of two nearly contiguous 

 pectinated laminae, united to each other below, enclosing 

 between them a narrow space, widening above, and allowing 

 the transmitted aqueous currents to pass over one free upper 

 margin, into the canal of the expiratory vent. As the conti- 

 guous sides of each lateral pair of gills are continuous at 

 their upper margin, where they are fixed, and at their free 

 margins below, the entire pair of one side in the conchifera 

 may be considered as only a puckered or folded condition 

 of the simple expanded reticulate membrane lining either 

 of the two sides of the respiratory cavity of a cynthia 

 or other tunicated animal. The compressed filaments which 

 compose the branchial laminae are slightly connected toge- 

 ther by regularly disposed tubercles, whose contiguous sur- 

 faces appear to be also provided with minute slow moving 

 cilia ; these component fibres of the branchiae are nearly free 

 in pecten, area, and spondylus, and are united only at their 

 distal ends in the malleus. The branchial laminae are some- 

 times unequal in size, as in cardium, where the exterior are 

 not half so large as the interior laminae. The branchial 

 currents of conchifera bring food to the mouth placed at the 

 bottom of the respiratory sac, they assist in removing the 

 excretions and the ova or embryos from the cavity of the 

 mantle, and they may aid in clearing or enlarging the per- 

 forations made by burrowing species. Small portions of the 

 branchial laminae, detached from the body, continue long 

 their ciliary movements, and present the most favourable 



