296 MOLLUSCA. 



marine animals (fig. 172). Some have a curious organ called an 

 "ink-bag,"' which contains a brown fluid, "sepia": this fluid 

 the mollusc ejects when pursuing its prey so as to confuse it, and 

 likewise to hide itself when being pursued by its enemies. The 

 Nautilus has a complete external shell, quite unlike the " cuttle- 

 boaes" given to birds, the internal shell of Sepia. The sexes 

 are separate, and some remarkable ways of reproduction exist 

 (hectocotylisation), but such subjects are outside our province. 

 Many fossil forms are found, as the Ammonites and Belemnites 

 of the Lias, Gault, and Chalk, &c. 



The Pteropoda are a small class of free-swimming pelagic 

 Mollusca provided with two wing-like appendages and with 

 glassy shells. They are carnivorous, feeding mainly upon Crus- 

 tacea, and they in their turn form part of the food of whales. 



Gasteropoda. — Gasteropods include the majority of univalve 

 Mollusca. They are found in the sea, in fresh-water, and on 

 land, The foot is in the form of a broad, flat, ventral mass, or 

 may be in the shape of a curious fin-like organ. Gasteropods 

 have never a bivalve shell — the shell is either univalve or multi- 

 valve. The simplest type of shell is seen in the limpet. They 

 are often coiled or " whorled " (fig. 173); each division of the 

 spire or whorl may be in contact or widely separate ; the mouth 

 is either round or oval, slit-like in a few genera {Cypi-cea), and 

 bordered by the rounded "lips." A column or piUar runs up 

 the centre of the spiral shells, forming the columella. The 

 body of a Gasteropod is divided into head, foot, and visceral sac; 

 the last is enclosed in the mantle, which is never divided into 

 two lobes as in bivalves. Most Gasteropods have an unsym- 

 metrioal body, coiled spirally, the respiratory organs of the left 

 side being atrophied. The flattened foot may secrete a horny or 

 calcareous plate, the operculum, whioli serves to close the orifice 

 of the shell when required. The head is distinct, with two ten- 

 tacles, and two eyes often placed upon long stalks. A proboscis 



