360 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 97 
(not nine) rings in front of anus, between the fifth and sixth muscle 
bands. No mention is made of the cerebral organ or of the peculiar 
dorsal crests of the muscle bands adjacent to the nephridia. The 
nephridia are described as being short and bright brown, but in S. 
nudus the color in preserved specimens varies from pale pink to deep 
brown and the length from short to long. In aspecimen from Florida 
the left nephridium is 80 mm. long and the right only 15mm. It is 
therefore unsafe to rely on dimensions for specific characters. A 
feature of galapagensis that may be of some systematic value is the 
attachment of the retractors to body wall. They do not fan out with 
a clear-cut posterior margin as if superimposed on the underlying 
muscle bands as in nudus. Instead, the deep grooves between the 
muscle bands of the body wall are continued for a varying distance 
upon the retractors, which are thus at their base subdivided into 
distinct fascicles. 
XENOSIPHON, new genus 
Diagnosis. —Differing from Sipunculus s. s. in the following par- 
ticulars: An extra pair of muscles, functioning both as retractors 
and protractors, arising from posterior border of introvert and in- 
serted in front of brain, rectum unusually long, the anus being in 
front of nephridiopores, postesophageal intestine without a long for- 
ward loop, spindle muscle arises from wall of rectum, nephridia long, 
slender, attached to body wall for nearly their entire length, squami- 
form papillae of the very short introvert increasing in size toward the 
tentacles, which have very many leaflets arranged in subtriangular 
pads surrounding the mouth, type species with papilliform dermal 
outgrowths. 
Genotype, Sipunculus branchiatus Fischer. 
Remarks.—This genus is instituted for the highly peculiar species 
described below. It cannot be determined whether the skin papillae 
are of generic significance since Spengel found the skin-canal system 
to vary considerably within the indicus group. The papillae may 
represent a further development to increase respiration surface. 
XENOSIPHON BRANCHIATUM (Fischer) 
PuaTE 12 
Sipunculus mundanus var. branchiatus W. FiscuEer, 1895, p. 3, pl. 1, figs. 1, 
la, 2. 
eon branchiatus SPENGEL, 1913, p. 74. 
Description.—Length, 310 mm., introvert and tentacle crown, 20— 
25 mm., thickness of cylindrical body, 8, 12 mm., this varying accord- 
ing to constriction of ring muscles. The specimen from La Paz, 
420 mm. long, is constricted in the middle of body to 8 mm. diameter. 
Longitudinal muscle bands 29-34 (Panama specimen, 32-34), only 
