Phylum Annelida and Other Wormlike Coelomates 465 



in early summer, and all of the individuals in an area spawn at the 

 same time. Thus fertilization is ensured. The fertilized eggs become 

 swimming trochophore larvae for some forty days before metamor- 

 phosing into the typical burrowing worms. 



The Phylum Sipunculoidea. — These saclike worms, sometimes 

 known as "peanut worms" are all marine forms which usually dwell 

 in shallow water (Fig. 151, C). Some live in sand, others under rocks, 

 and still others in empty burrows or shells of other animals. Most of 

 them are quite small, but one species attains a length of 18 inches. 

 Those dwelling under rocks tend to be slenderer than those that bur- 

 row in the sand or mud. 



The body of the sipunculids is unsegmented and lacks setae or 

 parapodia; at times a blood-vascular system is present. The single 

 pair of nephridia act as gonoducts and the sexes are separate. There is 

 a ventral nerve cord. The anterior portion of the body has a retractile 

 proboscis surmounted by tentacles. The digestive tract is complete 

 with both mouth and anus opening at the anterior end. This is a very 

 convenient arrangement for an animal that lives in shallow burrows, 

 for feces can be expelled easily. 



When feeding, the sipunculid spreads its tentacles over the surface 

 in which it is living. The cilia on the tentacles beat, causing a flow 

 of current toward the tentacles. Food which is brought in with these 

 currents is entangled in mucus. This food is then carried to the mouth 

 and swallowed. There is little or no selection of particles by the ten- 

 tacles, and small grains of sand are swallowed as readily as tiny ani- 

 mals. 



The eggs and sperm arise from the lining of the anterior portion 

 of the body. These are discharged into the water and a trochophore 

 larva develops from the fertilized eggs. After a brief swimming period, 

 these larvae metamorphose into the adults. Often just before the spawn- 

 ing period, the sipunculids gather in great numbers. 



In California, North Carolina, Key West, and Europe, the most 

 commonly seen sipunculid is Sipunculus niidus ; in California, the com- 

 mon peanut form, Dendrostoma petraeuni, lives beneath rocks. 



