MOLLUSCS 307 



Invertebrates. Each side of the ring is thickened into a ganglion, 

 from which a lateral nerve-cord runs along the corresponding side 

 of the body, while there is a much more slender nerve running 

 back in the middle line above from the upper side of the ring. 

 This dorsal nerve has attracted a great deal of attention, for it 

 has been compared to the spinal cord of a Vertebrate, though, 

 unlike this, it is solid. 



MOLLUSCS (Mollusca) 



Molluscs include such familiar shell - fish as Periwinkles, 

 Oysters, Cockles, and Mussels, as well as soft-bodied animals 

 like Cuttle- Fishes, to which the term Mollusca (Lat. mollis, soft) 

 was originally applied. 



Examination of such a typical form as the Ormer or Sea- 

 Ear {Haliotis tuberculata), which is common in the Channel Isles, 

 will give some idea of the characters of Mollusca in general, 

 and of the special subdivision to which this particular sea-snail 

 belongs (fig. 178). 



External Characters. — The most obvious feature is the 

 presence of a large external shell covering the upper side of the 

 body, from which it cannot be detached without cutting through 

 a large rounded fleshy mass, the shell-muscle. The shell is 

 not symmetrical, for a row of holes can be seen running along 

 near its left margin, and on the right side at the back a spiral 

 twist can be made out. Turning the animal over, a huge fleshy 

 mass with a flattened surface is seen projecting from the under 

 side of the body. By means of this foot the Ormer is able to 

 adhere to rocks like a Limpet, and to crawl about like an 

 ordinary Snail. A foot in some form or other is characteristic of 

 all Mollusca, and it must be understood that the word is here 

 employed with a special meaning of its own. Projecting in front 

 from above the foot a short head can be seen bearing a blunt 

 snout, at the end of which the mouth is placed, and a pair of 

 pointed feelers or tentacles, which are solid, and cannot therefore, 

 like those of a common Snail, be withdrawn into the body. Seen 

 from below, the body of the Ormer is bilaterally symmetrical, and 

 the twisted condition of its upper part is a special condition 

 characteristic of snail-like forms in general. 



On removal of the shell by cutting through the shell- 



