354 



THE ANIMAL KINGDOM 



Moull 

 Retractor 



TYiUScle 





Re-lraclilc 

 on 



Ne-pliridiam 



Iritesiiiie 



Ventral 



nerve cord" 



appendage 



Sipu"nculoidga 



Priapuloidea 



Figure 18.2. Three groups of marine worms. Sipunculoidea, cut open to show some 

 of the internal organs. (After Brown.) Echiuroidea (after Parker and Haswell) and Pri- 

 apuloidea (after Theel) shown in side view. 



and lie buried in the mud or widiin cavities of shells with a greatly 

 developed, mucus-covered prostomium projecting. The prostomium is 

 ciliated, and is used for gathering detritus from the bottom surface and 

 passing it to the mouth. Neither a distinct brain nor sense organs are 

 present; the esophagus is surrounded by a nerve ring continuous with 

 the ventral nerve cord. 



In the genus BonneUa, which has a very long, forked prostomium, 

 an interesting case of sexual dimorphism is found. Each larva can de- 

 velop into either sex. If it settles by itself on the bottom it becomes a 

 female, which is a sizable, fully developed worm. If the larva lands on a 

 female, however, it becomes a male, which remains microscopic in size 

 and simplified in morphology, and lives in the mouth or nephridia of 

 the female. 



Sipunculoids are elongate, flexible worms with a retractile an- 

 terior end used for burrowing in sand. They swallow the sand and 

 digest the debris and small organisms it contains. The mouth is sur- 

 rounded by a ciliated, tentacled disc. The digestive tract includes a 

 long intestine that doubles back from the posterior end to a dorsal anus 

 well forward on the body. The nervous system is well developed and is 

 similar to that of the annelids, but the circulatory system is reduced and 

 restricted to the anterior end of the body. The coelom is large and un- 

 divided. 



160. The Priapuloids 



The Priapuloidea (Fig- 18.2) is another phylum of sizable marine 

 worms that lack segmentation. The anterior end is retractile and carries a 



