ANATOMY OF PHASCOLOSOMA. 159 
pairs with their external slits arise, somewhat as in Ascidians. 
The entire Tornaria directly transforms into the worm, the 
transitional period being very short. The body lengthens, 
the collar and proboscis develop, and the worm eventually is 
as seen in Fig. 108; afterwards the body lengthens, the end 
tapering and becoming much coiled. 
Ciass VII.—ENTEROPNEUSTA. 
Footless, smooth-bodied worms ; with no bristles, a large exserted soft 
fleshy proboscis ; breathing by a series of dorsal respiratory sacs opening 
into the digestive canal, and communicating externally by spiracles; the 
nervous system situated above a notocord, (Balanoglossus.) 
Crass VIII.—GrErHyReEa (Star-worms). 
General Characters of the Gephyreans.—The most acces- 
sible type or representative of this small but interesting group 
of worms is a large, smooth, cylindrical worm from six to 
ten inches long, which is common in sand or sandy mud at 
low-water mark. It is the Sipunculus or Phascolosoma 
Gouldit Diesing, and from its abundance and large size, as 
well as the ease with which it can be preserved in spirits, is an 
excellent subject for the laboratory, serving as an example of a 
very aberrant type of worm as compared with the earth- 
worm, or with a Nereis. The body is as smooth as a pipe- 
stem, and about that size, unarmed, with a circle of numer- 
ous small, flat, foliaceous tentacles around the mouth. On 
laying open the body from the head to the extremity (Fig. 
110), the body-walls are seen to be lined with fine longi- 
tudinal flat muscles, with two unequal pairs of large white 
retractor muscles, the anterior third of the body being 
highly retractile. The intestinal part is found to float free- 
ly, though anteriorly attached to the walls by a few muscu- 
lar threads, in the capacious body-cavity, and is usually full 
of fine mud. The csophagus is long and slender, situated 
between the shorter pair of retractor muscles ; behind the 
