12 THEEL, NORTHERN AND ARCTIC INVERTEBRATES. I. SIPUNCULIDS. 
Phascolion THÉEL 1875. As a supplement to the Synopsis it may be stated that 
these often small forms of Gephyrea, which are accustomed to pass their life in tubes 
of worms or in shells of Scaphopods and Gasteropods, have a number of small digiti- 
form tentacles arranged round the mouth, that the skin has on it papille and often 
also, behind the middle of the body, a zone or girdle of triangular or horse-shoe- 
shaped brownish prickles or attachihg papille, and that hooks with a few exceptions 
are present. 
Contrary to the views of KOREN and DANIELSSEN (1877), I must persist in con- 
sidering Phascolion as a well defined genus, separate from the older genus Phascolosoma. 
SELENKA and other later investigators also maintain the same opinion. However, I 
cannot agree with SELENKA in regarding the Phascolosoma squamatum of KOREN and 
DANIELSSEN as a species of the genus Phascolion. In that respect I entertam quite 
different views, as will be gathered further on. Besides, I am uncertain whether such 
a form as, for instance, Phascolion manceps is really a true Phascolion or not. For 
my own part I can hardly believe it. According to SELENKA this remarkable species 
is devoid of hooks but has a single retractor; anus situated close behind the mouth 
and the vessel provided with side-branches. On the other hand, I agree most willingly 
with him When he says: "It seems extremely probable that the different species of 
the genus Phascolion do not form a compact, closely related, group, but that they 
have developed independently from various true Phascolosomata'" and "the genus 
Phascolion should be resolvable into several". 
In 1875 and 1877 KOREN and DANIELSSRIN described and figured two samples of 
an animal which they considered to represent Y new species of a new genus, which 
they named Tylosoma Liittkenii. Not having had the opportunity to investigate the 
types I base, the following reflections exclusively on an inspection and comparison of 
their text and figures. When doing so, I could not refrain from thinking that a cer- 
tain inconsistency was manifest. The most striking characteristic of the new genus 
ig said to be, that it lacks proboscis, tentacles and vascular system, and that the 
round oral aperture has its position in the centre of the truncated, broad scutiform, 
anterior part of the body. But how does the matter stand in reality? Of course it 
is impossible to produce full evidence, but a glance at Fig. 16 on Plate XIV has 
excited my suspicion as to a blunder having been committed. Thus, I am inelined 
to believe that that part, which is marked with the letters sp, represents a portion 
of the withdrawn proboscis and its nerve-cord, that tentacles really exist within the 
retracted proboscis-tube, and that the "round oral aperture'" is nothing but the ope- 
ning of the retracted proboscis-tube. With regard to the papille, they seem to bear 
the closest relation to those of Phascolion pallidum (KOREN and DANIELSSEN) =Phascolion 
tuberculosum THÉEL, and the "saucer-shaped'"" papille in the middle of the body are 
certainly nothing else but flattened and excavated rounded ones, common in the 
species just mentioned. 
But how are we to arrive at the real facts about the retractor-musceles? KOREN 
and DANIELSSEN Write: "One retractor, which takes its issue from the posterior extremity 
