STRTJCnmE AND PHYSIOLOGY OF THE MOLLrsCA. 27 



and forms a vaulted 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 ovulum volva, and forming a tubular 

 spine in typhis. 



In the plant-eating sea-snails {hoJostomata) there is no true 

 siphon, but one of the '* neck-lappets " is sometimes curled up 

 and performs the same office, as in paludina and ampuUaria 

 (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 fissurella, haliofis, and dentalium, 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 palvtdina, 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 Cuvierian 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 calamarios, "« hich 

 dart backwards with the recoil, like rockets. 



C2 



