364 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 97 
tween the skin and circular muscles is a system of parallel, longi- 
tudinal canals about 0.5 mm. in diameter in the posterior constricted 
region. They lie close together and apparently are not continuous 
between two annuli; air bubbles in the transverse, or circular, inter- 
muscular coelomic spaces (described below) can be forced into them. 
They are thus a series of peripheral, longitudinally oriented, sub- 
dermai coelomic spaces, fed by the intermuscular discontinuous ring 
canals, which in turn open into the coelom, between the longitudinal 
muscle bands. 
The anterior end of introvert is thin-walled and translucent. The 
first 20 mm. is armed with spines (pl. 13, figs. 3, 4) in about 28 circles. 
They decrease in size posteriorly and persist a little farther on the 
dorsal than on ventral side of introvert. The longest are 0.34 mm. 
and are nearly straight; sometimes the tip is curved slightly back- 
ward, or forward. The shorter posterior spinelets are usually curved 
backward (pl. 13, fig. 3). 
The tentacular crown is voluminous. There are eight pinnate 
groups. Plate 13, figure 5, is a diagrammatic plan of the furrows 
leading to the mouth. These are bordered on each side by five to 
seven grooved pinnae. It is the swollen, raised border of these pin- 
nae (not indicated in fig. 5) that gives the characteristic form to the 
tentacles of Phascolosoma (Théel, 1905, pl. 14), which these closely 
resemble. The two ventral tentacles are bifurcate. Between the two 
dorsal tentacles is the conspicuous nuchal organ, around the anterior 
border of which is a double fold of skin and a deep groove. 
The inner, longitudinal muscle layer is pale pink, of a satiny luster, 
and smooth throughout the long introvert, although wrinkled trans- 
versely. In the succeeding inflated region it is divided into unequal, 
freely and irregularly anastomosing flat bands. The narrow spaces 
between these bands are crossed by slender, subequal fascicles of 
the adjacent circular layer. The intervals between the circular bands 
(which anastomose) are the entrances to the transverse intermuscular 
lacunae, or discontinuous canals, which in turn feed the subcuta- 
neous system of lacunae. Along the fissures between the longitudinal 
muscles, irregularly distributed, are dark reddish, convex, ellipsoids, 
1-1.5 mm. long. A few are found on the muscles. 
In the constricted, annulated, posterior region, the longitudinal 
muscles only occasionally anastomose. They form conspicuous, nar- 
row, closely placed ridges 18 to 22 in number. By reason of their 
extreme contraction the circular muscles are very much thicker than 
anteriorly. Each annulus, marked by deep grooves, contains half 
of two bundles of circular muscles, because the deep constriction 
divides the muscle nearly in two (pl. 18, fig. 6). In the middle of 
the annulus is the transverse lacuna. The connecting outlets to 
