NEMERTINE TFORMS. 
457 
might be expected from their slender build, and the presence of a large horizontal 
tin at the sides of the hinder half of the body, projecting beyond the tail. The 
shape oi the body and the presence of the large tail-tin suggested the name Sagitta. 
The head is bluntly rounded and furnished with a pair of eyes, a pair of feelers. 
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 nse 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 END 0F PEO boscis or 
other organs of the body. The winding curled tubes, which Tetrastemma (enlarged), 
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 
