DIGESTIVE OllGANS. 93 



chiates, but wanting in Nautilus. The liver is alwa^'S large; and 

 the two hepatic ducts are generally glandular. A large and 

 sometimes spirally-wound csecvm is frequently developed from 

 the commencement of the intestine, with which the hepatic ducts 

 communicate. The product of the salivary glands is uncolored, 

 limpid and acid, whilst that of the liver and pancreas is also 

 uncolored and acid, but rarely limpid. 



In the tetrabrauchiata and the decapoda the oesophagus is 

 dilated into a C7-op separated from the stomach b}^ a constriction. 

 The csecum is small and rounded. and the intestine is twice bent 

 upon itself. The four-lobed loosely racemose liver is lodged in 

 the anterior portion of the perivisceral cavity, and is largely 

 developed. From either side of it spring biliary ducts which 

 open in the large blind-sack ; the pancreas is found at their 

 commencement. 



Gastropoda (ix, 96, 9^; xiv, t2, H, ^6; xv, 81, 78). The diges- 

 tive organs are well developed. The mouth, which is sometimes 

 in the lower plane of the head, and sometimes at the end of 

 a proboscis capable of protrusion and retraction (Ixii, 21), is 

 encircled by extensible lips ; within, it is often armed with one or 

 several jan^s (xi, 34, 36), and the tongue usually bears on its upper 

 surface numerous transverse rows of teeth, constituting the 

 lingual ribbon (x, 11). The oesophagus is often beset with 

 appendages and salivary glands, and leads to the stomach ; whence 

 the intestine turns forward, passing close to the kidney and heart 

 and into the respiratory cavit}^, the right side of which it traverses 

 and finall3^ proceeds to the anus. The intestine and often 

 a portion of the stomach is embraced b}^ an enormous liver, filling 

 nearly the whole of the first whorl of the shell, and pouring its 

 secretions into the former (and often into the latter also) by 

 several openings. We will successivelj'^ examine these various 

 organs more in detail. 



The proboscis is a production of the skin of the anterior or head 

 portion of the bod}^, bearing the mouth at its end. When it 

 remains permanently protracted it receives the name of rostrum, 

 that of proboscis being more properly limited to this organ when 

 provided with muscles by which it can be retracted^ within the 

 body. The t3^pical proboscis is quite characteristic of the siphon- 

 ostomated prosobranchiates. or those carnivorous mollusks of 

 which the shell is canaliculate or notched at its lower extremity ; 

 whilst those animals provided with a rostrum or snout, or with a 

 simple mouth are members of the usually phytophagous holos- 

 tomata. In Dolium, a remarkable exception, the exceedingly 

 long proboscis accompanies phytophagous habits. 



The invagination of the proboscis is eflected by means of 

 powerful retractor muscles supplied along its entire length and 

 especially numerous at its base, where the retraction begins. The, 



