KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 39. N:o |. 13 
with two strong roots, that form nearly half the length of the retractor, and of which 
one is twice as broad as the other. 'The retractor goes about in the middle of the 
perivisceral cavity to the anterior part of the oesophagus. The latter is narrow and 
long, accompanies the retractor and extends down to that place where the two roots 
of the retractor wnite.” As far as I can understand, it ought to be evident from the 
above reference, that we here have to do with an animal in possession of two un- 
paired retractors, a dorsal and a ventral. For it is impossible to conceive, that two 
retractor-muscles of very different sizes can belong either to the dorsal or to the 
ventral system. As a rule it is known with regard to the Gephyrea that the re- 
tractors forming a pair are exactly equal in size, and that also the two roots of a 
single retractor are equal. Consequently the retractors in question must be unpaired 
and represent a dorsal retractor and a ventral one. If that be the case, the retractor- 
muscles closely resemble those of the genus Phascolion, and it only remains to find 
out whether the ventral one has the base split, which, according to my view, would 
signify that, in fact, two ventral retractors are present, besides the dorsal one, as is 
the ease in Phascolion strombi and Ph. tuberculosum (= Ph. pallidum). 
The alimentary canal too, seems to bear resemblance to that of the two species 
mentioned. Moreover, there is only one segmental organ. 
Thus, for my own part, I am convinced that the separate genus Tylosoma will 
not survive criticism but must be relegated to the older Phascolion. Moreover, I am 
of the belief that the species Litkenii, too, is nothing but a contracted sample of 
Phascolion tuberculosum. 
Onchnesoma KOREN and DANTELSSEN 1875 and 1877. The following may be added 
to the characteristics given in the Synopsis: Tentacles few (about 10) and small, or 
else absent and replaced by a tentacular disk. Proboscis lengthened, more than 2'/. 
times longer than the body itself. Body pear-shaped. Skin nearly smooth or covered 
with brownish scales or minute merusted irregular darkish bodies, always with small 
papillae. Hooks missing. 
Among the three species described by the Norwegian naturalists as belonging 
to the genus in question, only one, viz. O. Steéenstrupui, can be retained. The other 
two, O. Sarsi and glaciale, are true Phascolosoma-forms with highly reduced tenta- 
cular crown. 
On the other hand, I refer to this genus, though with some hesitation, the 
Phascolosoma squamatum of KOREN and DANIELSSEN (= Phascolion squamatum of 
SELENKA), which in several important poimts seems to be related to the original Onch- 
nesoma Steenstrupii. Singularly enough neither KOREN and DANIELSSEN nor SELENKA 
observed the true position af the anus. I do not understand at all how it was pos- 
sible for the two Norwegian investigators to write: "The anal aperture round and - 
close to the commencement of the proboscis ..." Moreover, they have also figured 
it so — quite wrongly. 
Possibly also SELENKA's Phascolion manceps ought to be counted as a repre- 
sentative of this genus, its anal aperture having been removed far from its typical 
K, Sv. Vet. Akad. Handl. Band 39. N:o 1. 2 
