On the Form, Growth, and Construction of Shells. 25 



vegetable feeders (or Holostomata) (see Coloured Plate, Vol. x., 

 p. 241, Figs. 2, 3, 4), and notched, or produced into a canal 

 in the carnivorous families (or SiphonosiomataJ (see same 

 Plate, Fig. 7— and Plate, p. 21, Figs. 7—11). But this 

 caual, or siphon, is respiratory in its office, and must not, 

 therefore, be taken as a certain indication of the nature of the 

 animal's food. Thus, for example, Scalaria pretiosa (Plate, 

 p. 21, Fig. 2) has a holostomatous aperture, but is known 

 to be carnivorous in its diet. If we refer back to the figures 

 of the dog-whelk (Nassa reticulata, Vol. x., p. 247), and the 

 common whelk (Buccinum undatum, p. 248), we shall see the long 

 incurrent siphon protruding from the canal of the shell and 

 turned upwards. Into this tube the water passes, and enters 

 a vaulted chamber (formed by an inflection of the mantle of 

 the animal), which contains the pectinated, or plume-like gills. 

 After traversing the length of the gills, it returns and escapes 

 through a posterior siphon, generally less developed than the 

 anterior one, but very long in Ovidum volva, and formed into a 

 tube in Typhis. 



The object of the long siphon in the whelk is to enable it 

 to respire freely while burrowing in the sand in search of its 

 prey — the poor defenceless My a, and other bivalves. The 

 Ampullaria has also an extremely long siphon, which enables 

 it to breathe, when it retires deep beneath the river mud 

 during the dry season. 



In the ear-shell (Haliotis), found living on the rocks at 

 low-water in the Channel Islands and elsewhere, and so com- 

 mon a mantel-piece ornament, on account of its pearly interior ; 

 the excurrent siphon is accommodated by a hole near the lip 

 of the shell, repeatedly renewed with the growth of the 

 animal. In the key-hole limpet (Fissurella) , the anal siphon 

 passes through the perforation on the summit of the shell. 



In Siliquaria(see Coloured Plate, Fig. 10, Vol. x., p. 241), 

 the notch for this siphon remains unclosed, so that as the shell 

 grows, it prolongs the fissure through the whole length of its 

 tube. 



In those mollusks whose shell is reduced to a mere rudi- 

 mentary organ, we still find that its design and object is, 

 primarily, to protect the heart and breathing organs. Thus, 

 in Testacella (b'ig*. 4, Vol. x., p. 245) it covers the hemal, or heart- 

 region, and again in the keel-shell, Carinaria (see Plate, p. 

 21, Fig. 3), in which the shell is less than one-seventh part 

 the size of the body of the animal, its only use is to cover the 

 branchiae. 



Lamp-shells (Brachiopoda) . — These curious bivalves are 

 symmetrical in form, and nearly always have the dorsal valve 

 smaller than the ventral, the latter being produced into a 



