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THEEL, NORTHERN AND ARCTIC INVERTEBRATES. I. SIPUNCULIDS. 
With regard to the museles in question, the genus Phascolion forms in several 
respects an exception to the rule. Here a dorsal retractor still remains side by side 
with the ventral ones, and it is very probable that it is formed by thé coalescence 
of two primitively separate ones. The fact that Phascolion hedrexeum SELENKA has 
the dorsal retractor provided with two roots, indicates something of that nature. 
Whether there are really two dorsal retractors in the Phascolion lucifugax SELENKA, 
or two ventral ones, the attaching points of which are removed towards the dorsal 
side, must be left undecided. | 
In order to avoid arbitrariness in the systematic arrangement I think it needful 
to urge that the following survey of the disposition of the retractor-muscles be taken 
into consideration. 
1. Four retractor-muscles, two dorsal and two ventral, the former often slightly 
shorter, their insertion never at the posterior end of the body: Sipunculus, Physco- 
soma (with some very few exceptions), Phascolosoma (partly), Petalostoma and Den- 
drostoma (one specimen). i 
2. Three retractor-muscles, one dorsal and two ventral, the latter mostly joined 
together, except at their bases, which embrace the nerve-cord; their insertion in the 
immediate neighbourhood of the posterior end of the body: Phascolion (partly). 
3. Two ventral retractor-museles, not seldom wnited, except at their bases, which 
embrace the nerve-cord; their insertion near the middle of the body or more or less 
removed backwards, in some Aspidosiphon-forms as far as to the posterior end.: 
Phascolosoma (partly), Physcosoma (a very few species), Dendrostoma (except one 
specimen), Cloeosiphon and Aspidosiphon. (To judge from the figures and deserip- 
tions given by SELENKA, A. Cumingii and Klunzingeri must have two ventral retrac- 
tors, fully united, with the broad base erossing the newe-cord). 
4. One retractor, attached to the posterior extremity of the body: Onehnesoma 
(including Phascolosoma squamatum KOREN and DANIELSSEN) and Phascolion (three 
species, one of which at least is dubious as to its relationship). 
As to the manner in which the musecle-cells of the retractors change in shape 
in the differimg states of contraction and extension, there seem to exist some not- 
able peculiarities, which, so far as I am aware, have been overlooked in the Gephyrea. 
In this respect the muscular fibres of the body-wall itself apparently differ. 
Each retractor-musele is composed of a large number of muscular cells or fibres, 
which, like those of the body-wall itself, have the aspect of unstriated fibres. Ne- 
vertheless, when examined under a low magnifying power, such a muscle in the state 
of contraction presents the appearance of having been formed by striped muscle-cells. 
That is, however, not actually the case; consequently the matter deserves a special 
investigation, of a more thorough nature than I have had the opportunity of perfor- 
ming. However, the truth is, doubtless, that in the state of contraction the muscle 
exhibits a series of somewhat oblique and irregular, transverse bands or strie, in 
several cases resembling true striped musecles. When, on the other hand, the muscle 
is fully extended, the fibres are devoid of these apparent transverse "strie". When 
