ii8 



ZOOLOGY 



Fig. 78. — Introvert of a Hoplonemertine, 

 with stylet, "reserve" sacs, and 

 muscular bulb. After Hubreoht. 



A. Retracted. 



B. Everted. 



is produced into a certain number of lateral diverticula, but 

 these do not seem to be very definite in number or size 

 (Fig. 77). The stomach is lined by a layer of cells which 



are capable of assuming very 

 different outlines at different 

 times; they often break away 

 from the wall and are seen 

 floating in the lumen of the 

 digestive canal. There is no 

 special muscular coat, but some 

 of the muscle-fibres running 

 through the parenchyma are 

 attached to the walls of the 

 stomach. In most Nemertines 

 the alimentary canal is ciliated. 

 The anus is terminal. 



The most characteristic 

 organ in the Nemertines is the 

 introvert or proboscis, which 

 consists of the hollow eversible 

 anterior end of the animal. In its retracted condition this is 

 invaginated into a cavity, the lumen of the proboscis sheath, 

 just as the finger of a glove may be inverted into the glove. 

 The cavity of the proboscis sheath is a closed one, and full of 

 a corpusculated fluid; the walls of this cavity, i.e. the pro- 

 boscis sheath, are extremely muscular, when they contract 

 the pressure of the fluid drives the introvert forward and it is 

 everted. In many Nemertines the proboscis sheath extends 

 the whole length of the animal, and only ends just above the 

 anus. The method of the eversion of this proboscis is interest- 

 ing ; when it begins to protrude, it is the walls of the organ 

 which first grow forward, and the extreme end of the proboscis 

 — often armed with a spine — is the last part to appear, and 

 is therefore only to be seen when the proboscis is fully ex- 

 tended. It is retracted by a special muscle inserted into the 

 tip of the proboscis behind the spine, and arising from the 

 base of the proboscis sheath; when this contracts, the first 

 portion to disappear is the tip. The aperture through which 

 the proboscis appears is either terminal or ventral, but almost 



