1903. ] ANATOMY OF A NEW GEPHYREAN WORM. 39 
through sand or mud. Most species of Phascolion live with the 
posterior part of the body either embedded in mud or protected 
by some cast-off mollusk-shell; and if we assume, as the external 
habit of Phascolosoma teres seems to me to warrant, this similarity 
of mode of life between the two types, certain peculiarities of 
structure in Ph. teres become of easy explanation. 
Selenka, long ago, pointed out that those Sipunculids which 
live in shells usually have only one nephridium. Jn other words, 
in animals of sluggish habit, the needs of metabolisin are served 
by one excretory tube. 
Now in Ph. teres, while there are two nephridia, they are hoth 
small and are of different sizes; the left has retrograded to a 
greater extent than the right. It is to be noted that when only 
one nephridium persists—as in Phascolion—it is that upon the 
right side. 
Enfeeblement of the muscular system is correlated with inactive 
habits; and in worms lying half buried in mud a strongly 
developed system of retractors ceases to be a clamant need. The 
extreme case is reached in Phascolion, where often the ventral 
retractors alone remain, while an indication of regression of the 
dorsal retractor system is seen (vide supra) in Phascolosoma teres. 
Lastly, in this worm, the coils of intestine, completely filling 
up the sharply-delimited posterior part of the body, could scarcely 
be efficiently acted upon by any spindle-muscle, which, we see, is 
absent. 
In these structural characters, then, we seem to obtain in 
Ph. teres an indication of the path taken by Phascolion in its 
derivation from the more primitive Phascolosoma. 
Summary.— With the characters of the Group. 
PHASCOLOSOMA TERES, n, sp. 
Body 40 mm. long, introvert ca. 8 mm. Posterior portion 
stout, sharply demarcated, ellipsoidal, containing the coiled 
portion of the gut. 
Colour yellowish, merging to dirty bluish white over the 
posterior part. 
Skin smooth, iridescent, devoid of papille, translucent posteriorly, 
beset with numberless glands. A simple wreath of 15 tentacles 
surrounds the mouth. Forward on the introvert 10 rows of 
backwardly directed, slightly curved, brownish hooks, ‘(06-08 min. 
long. 
Two eye-spots. Alimentary canal free, without spindle-muscle 
or mesenterial attachment save close to the anus. Spiral winding 
absent, but the tube much coiled in its middle portion, Hind- 
gut very long. Twonephridia, the right the larger, one-twentieth 
of the body-length. Simple contractile vessel. 
Hab, Firth of Clyde. Dredged from 60 fathoms. 
