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withdrawn into the remainder of the body.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 papille 
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. : 
