106 UNDERWATER GUIDE TO MARINE LIFE 



RIBBON WORMS: Phylum Nemertea— F/^wr^ 33 



These are the lowest members of the animal kingdom to have a circulatory 

 system through which blood flows and a digestive system with a separate mouth 

 and anus. Both of these characters are of immense importance, allowing animals 

 to be more independent in their environment. The presence of blood, by which 

 food and oxygen are distributed throughout the body, and an anus, which frees 

 the mouth of the task of egesting undigested food, gives greater freedom of 

 action to an animal by increasing general body efficiency. 



The seas contain 550 species of nemerteans, which range from an inch to 

 100 feet long. The long ones, however, are not much thicker than a piece of 

 ordinary string. On the head end is a sheath bearing a long noselike proboscis. 

 At the tip of the proboscis a sharp stylet which resembles the nematocysts of 

 jellyfishes is usually present. The proboscis is shot out at prey hke an entangling 

 lasso, and those nemerteans which have a stylet use it to poison the prey. 

 Worms, fishes, or crustaceans may be captured and are swallowed whole. The 

 proboscis may also be used for defense and for burrowing. 



Nemerteans are found all over the world from the arctic to tropic seas. The 

 largest in North America is the flesh-colored Cerehratulns of the temperate east 

 coast. It reaches a length of 20 feet and burrows in sandy or muddy shores by 

 day, but it may swim about at night near the bottom. It is very common and is 

 often dug up by clam diggers. Other nemerteans are as numerous as Cerehra- 

 ttihis, and some are very beautifully colored with bright red or green stripes and 

 bars. A few species are pelagic. 



ARROW WORMS: Phylum Chaetognatha 

 ("bristle- jawed") — Figure 33 



These are swift-swimming, predatory, planktonic worms which reach the small 

 size of only about % of an inch and are extremely common in cold temperate 

 and arctic waters. They capture small planktonic animals in their bristly, 

 hooked jaws. There are about thirty-five species. Sagitta is the most common 

 genus. Their evolutionary position is not known. 



ROTIFERS: Phylum Rotifera ("wheel-bearer")— Figure 34 



These little animals are extremely interesting to watch, but unfortunately a 

 microscope is needed since their size is about that of a large protozoan. Some 

 rotifers have the most extraordinary shapes and are very common attached to 

 objects in shallow water. Some are pelagic. 



MOSS ANIMALS: Phylum Bryozoa ("moss animal")— Figure 34 



Some of these little plantlike animals are so similar to the hydroid coelen- 

 terates that the two groups can only be distinguished by an expert with a 

 microscope. As their name implies, they look mossy. They are colonial. Some 

 are erect and branching, and others are flat and encrusting in form. All are 

 enclosed in an external skeleton, which gix'es the colony support and may be 



