186 MARINE INVERTEBRATES 



extent of one sixth of the entire length of the worm, is capable of being 

 withdrawn into the remainder of the body. This portion is called the 

 introvert. The introvert is retracted by means of special muscles form- 

 ing a sheath around the gullet and connected at the other end to the 

 body-wall about half-way down the body. It is narrower than the rest 

 of the body and is covered more or less closely with small horny papillae 

 which turn backward and overlap like scales. The rest of the body is 

 divided into longitudinal furrows and circular markings, giving it the ap- 

 pearance of being divided into squares. These markings correspond to 

 muscles which lie beneath. The introvert, when retracted, leaves at the 

 opening a lobed and plaited fold of the integument, giving the appear- 

 ance of tentacles. When the introvert is expanded there is a fringe-like 

 funnel around the mouth. The body is covered with a horny cuticle 

 and has an iridescent luster. The animal lives buried in the sand and 

 feeds upon sand, deriving its nourishment from the organisms contained 

 therein. The sipunculoids pass so much sand and mud through their 

 bodies that they are said to modify the mineral substances on the bottom 

 of the sea, as earthworms do the soil of the land. 



GENUS Phascolosoma 



P. Gouldii. Body cylindrical, a foot or more in length, a quarter or 

 half of an inch in diameter when expanded ; but the body constantly 

 changes in size and shape as it is contracted or expanded ; surface of 

 body parchment-like in texture and marked off in small squares ; color 

 dull white to light brown. It is found on the New England coast in 

 sand and gravel at low-water mark. 



