STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 27 
and forms a yaulted chamber over the back of the neck, in 
which are contained the pectinated or plume-like gills (Fig. 68). 
In the carnivorous gasteropods (siphonostomata) the water 
passes into this chamber through a siphon, formed by a pro- 
longation of the upper margin of the mantle, and protected by 
the canal of the shell; after traversing the length of the gill, it 
returns and escapes through a posterior siphon, generally less 
developed, but very long in ovulwm volva, and forming a tubular 
spine in typhis. 
In the plant-eating sea-snails (holostomata) there is no true 
siphon, but one of the ‘‘neck-lappets’”’ 1s sometimes curled up 
and performs the same office, as in paludina and ampullaria 
(Fig. 109). The in-coming and out-going currents in the 
branchial chamber are kept apart by a valve-like fringe, con- 
tinued from the neck-lappet. The out-current is still more 
effectually isolated in jisswrella, haliotis, and dentaliwm, where 
it escapes by a hole in the shell, far removed from the point at 
which it entered. Near this outlet are the anal, renal, and 
generative orifices. 
The cephalopods have two or four plume-like gills, sym- 
metrically placed in a branchial chamber, situated on the under- 
side of the body; the opening is in front, and occupied by a 
funnel, which, in the nautilus, closely resembles the siphon of 
the paludina, but has its edges united in the cuttle-fishes. The 
free edge of the mantle is so adapted that it allows the water to 
enter the branchial chamber on each side of the funnel: its 
muscular walls then contract and force the water through the 
funnel, an arrangement chiefly subservient to locomotion.™ 
Mr. Bowerbank has observed that the eledone makes twenty 
respirations per minute when resting quietly in a basin of water. 
In most instances, the water on the surface of the gills is 
changed by ciliary action alone; in the cephalopods and salpians 
it is renewed by the alternate expansion and contraction of the 
respiratory chamber, as in the vertebrate animals. 
The respiratory system is of the highest importance in the 
economy of the mollusca, and its modifications afford most 
valuable characters in classification. It will be observed that 
the Cuyierian classes are based on a variety of particulars, and 
are very unequal in importance; but the orders are characterised 
by their respiratory conditions, and are of much more nearly 
equal value. 
* A very efficient means of locomotion in the slender pointed calamaries, a hich 
dart backwards with the recoil, like rockets. 
2 
