14 THÉEEL, NORTHERN AND ARCTIC INVERTEBRATES. I. SIPUNCULIDS. 
position, almost to the neighbourhood of the mouth, its single retractor being attached 
to the posterior end of the body-cavity, and the hooks being totally absent. Never- 
theless, it ought to be kept in mind that there really exist some diserepances among 
the three forms mentioned with regard to the tentacles. 
In 1883 LEVINSEN"' thought himself entitled to unite two genera, the Onchne- 
soma of KOREN and DANIELSSEN (1875 and 1877) and the Petalostoma of KEFERSTRIN” 
(1862 and 1865) under the older name of Petalostoma. Very little reflection will 
show the unreasonableness of such an idea. It is enough, for instance, to compare 
Keferstein's own diagnosis of the genus and its sole species. TI confine myself here 
to stating that, according to KEFERSTEIN, Petalostoma minutum has: "Zzweli grossen, 
soliden, blattförmigen Tentakeln"" -— two "Segmentalorgane kurz, ganz frei — "vier 
Retractoren, die sich dicht bei emander im hintern Drittel des Körpers ansetzen"" ete. 
If once it should be proved that the diagnosis of Keferstein is wrong in that 
respect, that there exist only two retractors in Petalostoma instead of four, then I 
should propose either to erase that genus-name and to incorporate its species in the 
"abyssorum-group'" of Phascolosoma or possibly to exelude that group from the genus 
Phascolosoma and to incorporate it in the genus Petalostoma. 
Synonyms. 
Sthephanostoma DANIELSSEN and KOREN 1881. = Phascolosoma LEUCKART. See above 
under Phascolosoma. 
Tylosoma KOREN and DANIELSSEN 1875 and 1877. = Phascolion THÉEL. See above 
under Phascolion. 
Phallosoma IRVINSEN 1883. = Sipunculus LInnÉ. See above under Sipunculus. 
Petalostoma LEVINSEN 1883. = Phascolosoma LEUCKART. Petalostoma KEFERSTEIN 1862 
and 1865 and Onchnesoma KOREN and DANIELSSEN. Nee above under Onch- 
nesoma. 
Some Anatomical and Systematical Remarks. 
It has been considered that the tentacles, the hooks behind them, and the re- 
tractor-musecles play an important part in the system of the Sipunculidea. It may 
be so as a rule, but in some cases their systematic importance may be problematical 
or non-existent. In connection with an explanation of these matters I intend in a 
few words to treat of the sexual organs and the segmental organs, both described by 
KOÖprrct 
> Nachrichten d. K. Ges. d. Wiss. Göttingen 1862 and 1865, and Zeitschr. f. wiss. Zool. XII and 
XV. 1862 and 1865. 
