Page:Natural History, Mollusca.djvu/164

152 the same structure as the whole alimentary canal, and is generally plaited in a longitudinal direction."

M. De Blainville considered that the organ of respiration in the Limpets was a vascular network, spread over the interior of a cavity, situated above the neck, with a wide opening in front. Hence he constituted the family into an order, which he named Cervicobranchiata, or neck-breathers. The opinion of so eminent a zoologist, adverse as it was to the received judgment of his fellow labourers in science, demanded a close investigation, which has been given by M. Deshayes and others. The result has been to show that such a cavity exists, with a structure similar to that of the Limpets, in many other Gasteropoda, which possess distinct and undoubted gills of the pectinate form, which I shall presently describe; and that there is no sufficient reason for believing that this chamber in the neck has any respiratory function at all: this office being fulfilled by the fringe of floating leaflets that encircle the body, as had been maintained by Cuvier and others.

In order to fit these little organs for the office which has been assigned to them, they are furnished with a multitude of cilia, microscopically minute, covering all parts of their free surface. By means of the constant undulating movements of these cilia, a perpetual current of sea-water is made to roll along each leaflet, communicating the requisite oxygen to the blood-vessels, of which it is mainly composed. The currents flow from the outer towards the inner edge, across the surface of each leaflet.