NEMERTINE WORMS. 



457 



might be expected from their slender build, and the presence of a large horizontal 

 fin at the sides of the hinder half of the body, projecting beyond the tail. The 

 shape of the body and the presence of the large tail-fin suggested the name Sagitta. 

 The head is bluntly rounded and furnished with a pair of eyes, a pair of feelers, 



ARROW-WORM (25 times nat. size). 



and an armature of close-set horny teeth, all of which must be of the greatest 

 service to the animal in its roving, predaceous life. 



THE NEMERTINE WORMS, Class Nemertinea. 



All the members of this group are characterised by having the body elongate 

 and flattened, at least on the abdominal 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 with 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 obscurum) in the illustration on 

 this page, and retracted within the body in the full illustra- 

 tion of the animal on p. 458. The creature thrusts forth its 

 proboscis with lightning speed at passing animals, such as 

 crustaceans. The figure of T. obscurum illustrates other 

 characters in 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 backwards from each to the hinder end of 

 the body is a long nerve-chord, 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 pre- 

 sence of four eyes, are widely distributed, most of the species being minute, 

 and commonly 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 



END OF PROBOSCIS OF 



Tetrastemina (enlarged). 



