I certainly agree with the other folks that this worm is a sipunculan, and as Gisele points out, it is likely a species of Phascolion.
As for the terminology used in several responses to your question, I'd like to use this opportunity to point out a couple of issues. In the same way that we refer to polychaetes within a family (e.g. pectinariid from the family Pectinariidae; maldanid from Maldanidae; etc . . ), the usage of the name 'sipunculid' should refer to species within the family Sipunculidae. Phascolion, if this is indeed the correct identification, is a member of the family Phascolionidae.
>From Cutler, 1994; pg. 2: "The current spelling of the phylum, Sipuncula, and the use of "sipunculan" for the vernacular name (not sipunculid) was proposed by Stephen (1965) and restated be Stephen and Edmonds (1972)."
>From Stephen and Edmonds, 1972; pg. 3: "Two common names. 'sipunculoid' and 'sipunculid', exist for an animal that belongs to the phylum. Neither seems to be entirely satisfactory, 'sipunculoid' because the term Sipunculoidea should refer to a superfamily and 'sipunculid', as Hyman (1959 : 611) points out, because it is the name of an animal that falls within the family Sipunculidae, a family already proposed by Baird (1886 : 77). If the phylum is to be called Sipuncula it seems reasonable and logical to use the term 'sipunculan'. If the new term is adopted, the confusion that exists over the use of the term sipunculoid and sipunculid will be avoided."
Final note: Danny provided a very helpful schematic and image to help recognize a sipunculan in the genus Phascolion. You will see the term "proboscis" in the schematic. Sipunculans do not have a proboscis, which we know is quite specific in annelids . . . the correct term for what is likely referred to in the schematic is 'introvert.'
Just some, hopefully, helpful thoughts!
Michael J Boyle
Smithsonian Marine Station
701 Seaway Drive
Fort Pierce, FL 34949