644 MOLLUSCA-CLASS V.— CEPHALOPODA. 



peristome is sharp. The cellar-snail, Vitrea cellaria, is one of several British 

 examples. 



The Testacellid^ and Selbnitid^ are the most highly specialised of the 

 Stylommatophora and are almost all voracious carnivores. Testacella looks 

 like a slug with a tiny shell at the end of its tail. It is a worm-eater and 

 follows its prey in their burrows, hence it is seldom seen though fairly 

 abundant in England. Other members of the group have shells of varying 

 sizes and shapes. 



CLASS v.— CEPHALOPODA. 



Tub Cephalopoda include not only the most highly organised Mollusca, but 

 also the largest of them all, some forms, it is calculated, exceeding 50 ft. in 

 total length. The pearly nautilus has an external shell, the cuttle-fish and 

 its near allies have internal shells, whilst the octopus has none at all. 



The Cephalopoda are symmetrical animals, the two halves of the body 

 corresponding in structure. The visceral mass is large and often elongated. 

 The head, on either side of which there is a large and well-developed eye, 

 is more or less distinct, and is surrounded by the foot, which has in fact 

 grown around the head and has developed, or been drawn out into, eight or 

 ten long processes called arms. 



In the majority of living foims thefe aims are furnithcd with rows of 

 suckers or hooks. The upper part of the back of the foot has grown out into 

 two fleshy lobes, the free, outer edges of \\hich can be applied to each other 

 so as to form a tube, known as the funnel, or siphon, that con municates with 

 the mantle-cavity. Sometimes the edges of these lobes have grown together 

 and form a complete tube. The bell-shaped mantle which envelopes the 

 body is extremely tough and muscular. 



The mantle-cavity is on the under side towards the back of the body, and 

 the water for respiration is taken in at its open end and then discharged 

 through the siphon. By the forcible expulsion of water in this way these 

 animals can dait backwards through the water with great rapidity. 



The mouth is placed in the centre of the arms; it is furnished with two 

 jaws which resemble a parrot's beak and which are moved by powerful muscles. 

 The radula is small in proportion to the animal, and has lut few teeth in 

 each row; the alimentary canal runs in an almost straight line from the 

 mouth to the stomach near the further end of the body, thtnce the intestine 

 leads forwards and downwards and terminates in the mantle-cavity. Just 

 at the same point (he ink-bag also opens into the mantle-cavity. By dis- 

 charging the ink or sepia-p'gment containtd in this gland, and thus clouding 

 the water, the animal is enabled to cover its retreat from an enemy. On 

 either side of the median line, within the pallial-cavity also, are the gills, 

 either two or four in number and foliobranchiate in type. The heart has a 

 corresponding number of auricles and a ventricle, whilst the blood is largely 

 confined to veins and arteries, so that nearly the whole of the blood return- 

 ing from the body passes through the gills. The nervous system is highly 

 concentrated, whilst the ganglia around the cesophagus are fused together 

 into a ring of nervous tissue. This nerve-collar in Nardilus is partly, and in 

 the other Cephalopoda completely, enclosed by a cartilage called the cephalic 

 cartilaye. There are other cartilages present, except in Nauiilvs, in different 

 parts of the body, that serve as points of attachment for the larger muscles. 



The faculty which Cephalopods possess of changing their colour is one of 



