362 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 97 
groups of tentacles, which probably increase in number as growth 
proceeds. 
The anus is equivalent of about five muscle rings (not clearly 
differentiated) behind the posterior papillae of introvert. 
Interiorly the longitudinal muscles form flat bands becoming angu- 
lar in section only when the body is much constricted. The introvert 
and four retractors occupy about one-seventh of the body length. 
The retractors are free from one another and arise at approximately 
the same level: Both ventrals from muscles 1-4, while both dorsals 
arise obliquely from muscles 7-11. The two protractors arise from 
muscles 12-15 at the posterior border of the introvert. Before in- 
sertion, 4 mm. in front of the brain, they pass over the dorsal re- 
tractors. Their form and position when the introvert is out and in 
are shown in plate 12, figures 1 and 2. The rectum passes far forward 
and opens close behind the (dorsal) origin of the protractors. (Mus- 
cles 17 and 18 are the two middorsal in figure 1; 18 and 19 are really 
17 and 16 of the left side.) The rectum lacks a thin fan-shaped 
muscle. A very slender spindle muscle arises from the ventral wall 
of rectum, 20-24 mm. behind anus, and proceeds backward following 
the gut; 10-12 mm. from its origin is a very small coecum to which 
it is attached. The rectum is fastened dorsally to the body wall by 
a continuous mesentery, as far back as the two lateral anchors just 
behind the origin of the spindle muscle. These short lateral strands 
of tissue fan out slightly and may be rudiments of the rectal fan- 
muscle. At any rate, to them is attached the ends of a delicate 
filament, forming a loop, which on each side passes obliquely ventral- 
ward along the origin of the dorsal retractors. Here the thread is 
thickly beset with delicate racemose structures (poorly preserved). 
These quickly thin out posterior to the muscles, and the rather long 
posterior loop is very delicate, translucent, and more loosely attached 
to the coelomic epithelium. Probably the ‘‘bandférmiges Organ” 
figured by Selenka (1883, p. 109, pl. 12, fig. 174 y) in S. mundanus 
is a fragment of a similar structure. It resembles a gonad but may 
be a more extensive ‘‘Zottenbildung.” 
The alimentary canal is macerated, but it appears to lack the 
forward loop which complicates the anterior end of the spiral of S. 
polymyotus. Although in plate 12, figure 1, the esophagus is drawn 
to the right, it naturally turns to the left, for its first attachment to 
the dorsal wall is by separate frenula, along muscle 9 (or 8) of the 
left side. The mesentery between the esophagus and the left dorsal 
retractor extends posteriorly only about half as far as the right. 
This shorter left mesentery allows the ventral vessel to become 
sinistral, while the dorsal vessel gradually becomes dextral. Both 
end dorsolaterally at the beginning of the dorsal frenula. From 
