152 CYCLOBRANCHIATA.—PATELLADZA. 
the same structure as the whole alimentary canal, 
and is generally plaited in a longitudinal direc- 
fon, 
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 un- 
doubted 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 en- 
circle 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 fur- 
nished with a multitude of célia, microscopically 
minute, covering all parts of their free surface. By 
means of the constant undulating movements of 
these célia, a perpetual current of sea-water 1s made 
to roll along each leaflet, communicating the requi- 
site 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. 
* Johnston: Introduction to Conchology, 328. 
