21« 



LOWER INVERTEBRATES. 



tracted proboscis lies, are situated in front on the under side; at the front also, but 

 upon tlie upper side, are placed four groups of eye-specks, but in most forms there are 

 only two such groups. The proboscis occupies the centre of the body, and is the 

 most conspicuous of the internal organs ; in it may be distinguished the stylet, which 

 in the cxserted proboscis projects from the tip. The presence or absence of this stylet 

 serves to distinguish the two sub-classes into which this group of worms is divided ; for 

 the Enopala, to which the Tetrastemma belongs, are furnished with a stylet, while the 

 Anopla are without. The second sub-class includes the majority of the better known 

 and larger nemertean worms. 



Fig. 205. — Polia crucigera. 



Muddy bottoms, botii in deep water and between the tides, yield to the digger 

 many species of this class. Perhaps the most remarkable of these thus obtainable is 

 the giant Cerebratulus ingens, which is met with under stones where there is sand. It 

 is an enormous, smooth, flattened worm, yellowish, whitish, or flesh-colored, and some- 

 times grows to be ten or twelve feet long and over an inch in width. C. rosea also 

 occurs in similar places ; in form it resembles its larger relative, but is less flattened, 

 smaller, and decidedly red in color; it is often covered by adherent gr.ains of sand. 

 Another species belonging to the nemerteans is also found on the Xew England coast 

 in great abundance under stones from mid-tide to high-watei- mark ; it is the JSfemertes 

 socialis, a gregarious species, many of them coiling together into large clusters ; it is 



