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. 



CLASS VIL-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 syxtem situated above a notocord. (Balanoglossus.) 



CLASS VIII. GEPHYREA (Star-worms). 



General Characters of the Gephyreans. The most acces- 

 sible type or representative oi 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 Pliascolosoma 

 Gouldii 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 oasophagus is long and slender, situated 

 between the shorter pair of retractor muscles ; behind the 



