3o6 



CHARACTERS OF INVERTEBRATE ANIMALS 



If now the finger be pushed out, it will represent the extended 

 proboscis. The pulling in is effected by means of a muscle band 

 which runs along the interior of the thread and is attached to 

 its tip, while the pushing out is the result of fluid being squeezed 

 into the thread from its sheath. This kind of principle is utilized 

 elsewhere in the animal kingdom, and a very familiar example 



PROBOSCIS PORE 



PROBOSCIS IN SHEATH 



iLATERAL NERVE 



Fig. 177. — Structure of a Nemertine (diagrammatic) 



A, Side view, internal organs seen by transparency. B, View from above affront end; position of mouth 

 and beginning of gut indicated by the dotted line, c, Cross-section. 



is found in the " horns " of the common Snail, which can either 

 be stretched out (as immortalized in the nursery rhyme wherein 

 the adventures of certain snail-hunting tailors are set forth) or 

 withdrawn into the body at will. 



Reasons have been adduced for thinking that the proboscis- 

 sheath is comparable to a notochord, and the proboscis to a 

 curious little structure attached to the under side of the brain in 

 Vertebrates, and known as the pituitary body. 



The central nervous system of a Nemertine is interesting in 

 many ways. It consists of a nerve-ring, which encircles the front 

 end of the proboscis, and not the digestive tube as is usual among 



