3430 



THE WORM-LIKE ANIMALS 



m 



IS 



THE NEMERTINE WORMS Class Nemertinea 



All the members of this group are characterized by 

 having the body elongate and flattened, at least on the ab- 

 dominal side; at the front end there are frequently two 

 clusters of eyes, and two apertures, one of which leads into 

 the alimentary canal, and the other into a cavity containing 

 a peculiar organ known as the proboscis. The latter, which 

 is used as an instrument for prehension, can be thrust out 

 with swiftness to a considerable distance, and in many 

 species is armed in the middle by a 

 sharp spike-like tooth and some smaller 

 ones at the sides, which are brought 

 by degrees into use as the large median 

 one is worn away or fractured. This 

 organ is shown protruded in a small 

 marine species (Tetrastemma obscu- 

 rum} in the illustration on the right, 

 and retracted within the body in the 

 full illustration of the animal on the 

 left. The creature thrusts forth its 



proboscis with lightning speed at END OF PROBOSCIS OF 

 passing animals, such as crustaceans. Tetrastemma (enlarged). 

 The figure of T. obscurum illustrates 



other characteristics of the anatomy of these animals. The 

 two swellings situated in the head end and united by a 

 cross bridge constitute the brain or chief centre of the 

 nervous system; running backward from each to the hinder 

 end of the body is a long nerve cord, supplying the muscles 

 and other organs of the body. The winding curled tubes, 

 which also run the length of the body, are the so-called 

 water vessels. These worms, which have received their 

 generic name from the presence of four eyes, are widely 

 distributed, most of the species being minute, and com- 

 monly found among seaweed. The worms of this group 

 that have hitherto been discussed have the proboscis armed 

 with stylets, and are consequently called the Hoplophora, 

 or armed nemertines. The second division, namely, the 

 Anopla, comprising those kinds which have no spines upon 

 the proboscis, contains some of the largest species of the 

 class, Meckelia somatotoma reaching a length of from three 



A FOUR-EYED NEMERTINE, tO over si ^ ^^ Jt is a long ' flat ' whitish-colored crea- 



Tetrastemma obscurum. ture occurring on muddy ground and between the branches 



(Enlarged.) of coral; and has received its name from the habit of break - 



