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. mo His, soft) 

 was originally applied. 



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

 Ear (Haliotis tuber culatd), 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 like those of a common Snail, be withdrawn into the body.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- 



