X. — MOLLUSCA. 75 



X. 



MOLLUSCA. 



The MOLLUSOA constitute one of the principal divisions 

 of the animal kingdom, and include such animals as the Octopus, 

 Cuttle-fish, iSquid, Snail, Slug, Whelk, and Oyster. 



They may be characterized as soft cold-blooded animals, with- 

 out distinctly marked external segmental divisions into segments 

 (as in worms) ; their cerebral ganglia lie above the commencement 

 of the oesophagus, and are connected with the inferior ganglia by 

 nerve chords. The heart consists of two or more chambers, 

 is situated on the dorsal side of the animal, and drives the blood 

 into spaces between the various organs of the body. Only the 

 Cephalopods possess an internal cartilage, but none have a bony 

 skeleton ; in the majority this is compensated for by an external 

 hardened shell which forms an outer covering to the animal, and is 

 called the mantle. The mantle either forms a free fold on each 

 side of the body, or is largely attached to the body-wall, as 

 in the Snail, or the Slug, and so gives rise to an air-chamber, 

 which, when its walls are richly supplied with blood, serves as a 

 lung. The ventral surface of Molluscs is produced into the so- 

 called " foot," which is very variously modified. The foot may be 

 more or less hatchet shaped, or curved and capable of serving 

 as a leaping organ, or sole-shaped and adapted for creeping ; its 

 margins may be produced into elongated processes, as the so-called 

 arms of the Octojjus, or of the Nautilus ; or, as in other Cephalo- 

 pods, another part of the foot may fold over and form a funnel, 

 through which the water used for respiration is driven outwards, 

 causing the animal to move in the opposite direction — this part of 

 the foot having therefore still the function of an organ of loco- 

 motion. By means of their muscular foot, the Razor-shells 

 (Solenidce), burrow in the sand, the Pond Snails (Limnceidcd) 



