291
be accompanied by partial rotation alongside the body axis and a bending relative to
the trunk (Hammond 1966; Herlyn and Ehlers 2001). Reversal of the process results
from relaxation of the “eversion muscles” and contraction of an retractor muscle.
The presomal portion of the latter is termed proboscis retractor whereas the metasomal portion is commonly referred to as receptacle retractor (Fig. 8.4a). The proboscis retractor consists of circularly arranged longitudinal muscular strands
(Figs. 8.4 and 8.6a, b) that anastomose and thus form a tube-like contractile mesh.
The mesh can display a complicated folding in cross section depending on the
taxon investigated (e.g., Dunagan and Miller 1991; Herlyn 2002). However, in all
extant acanthocephalans the muscle inserts anteriorly at the inner site of the body
wall just beneath the proboscis apex. After having extended through most of the presomal (part of the) body cavity, the muscle splits into two or three portions that separately pass through the bottom of the muscular apparatus suspending the cerebral
ganglion (Fig. 8.4a). The separate portions continue through the metasomal (part of
the) body cavity before they attach to the inner surface of the metasomal body wall
(Fig. 8.4a) (Herlyn 2017; Herlyn and Taraschewski 2017 and references therein).
Eversion and inversion of the presoma are usually repeated until the worm is
attached to the intestinal mucosa or deeper (Hammond 1966; Aguiar et al. 2018).
Characteristic for the resting position is a subsequent slight withdrawal of the neck
by contraction of the neck retractor, which also has a mesh-like organization
(Figs. 8.4a and 8.6c, d). As some of its strands enclose the lemniscs, usually paired
processes of the presomal tegument extending into the metasomal (part of the) body
cavity (Figs. 8.4 and 8.6d), contraction of the neck retractor presumably presses fluid
from the lemniscs into the presomal tegument, thus stiffening the everted proboscis.
Thereby, the transportation of fluid takes place via a so-called lacunar system
extending through tegument and lemniscs (e.g., Fig. 8.6c; Hammond 1966; Herlyn
2002, 2017; Herlyn and Taraschewski 2017). This may be the case or not, but it is
almost certain that there is no connection between the lacunar system inside the
tegument and lemniscs and any sub-tegumental structures including the body wall
musculature (compare Nielsen 2012). In addition, the musculature is not hollow,
although it may occasionally appear hollow due to preparation artifacts (for a discussion, see Herlyn and Taraschewski 2017; see also Nikishin 2004).
From a paleoparasitological point of view, the practical value of the above details
on the functional morphology of acanthocephalan anchoring is currently limited. In
principle, however, musculature and other decay-prone structures can fossilize
under certain conditions (Parry et al. 2018). In addition, most of the aforementioned
muscles are visible in total preparations of acanthocephalans (e.g., Fig. 5a, b in
Herlyn and Taraschewski 2017). Accordingly, the one or other of the aformentioned muscles might shine through the body wall of the yet to be discovered fossilized thorny-headed worm, comparable to the presumed alimentary tract in I. fellatus
(see Fig. 2a in Cong et al. 2017). Thus, if the fossil of a suspected endoparasite ever
discloses details of its internal organization, then a broader longitudinal strand
extending through the anterior body section (presoma) could correspond to the proboscis retractor of an acanthocephalan. Furthermore, strands with smaller diameter
that extend through the foretrunk (anterior portion of metasoma) could represent the
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…
be accompanied by partial rotation alongside the body axis and a bending relative to
the trunk (Hammond 1966; Herlyn and Ehlers 2001). Reversal of the process results
from relaxation of the “eversion muscles” and contraction of an retractor muscle.
The presomal portion of the latter is termed proboscis retractor whereas the metasomal portion is commonly referred to as receptacle retractor (Fig. 8.4a). The proboscis retractor consists of circularly arranged longitudinal muscular strands
(Figs. 8.4 and 8.6a, b) that anastomose and thus form a tube-like contractile mesh.
The mesh can display a complicated folding in cross section depending on the
taxon investigated (e.g., Dunagan and Miller 1991; Herlyn 2002). However, in all
extant acanthocephalans the muscle inserts anteriorly at the inner site of the body
wall just beneath the proboscis apex. After having extended through most of the presomal (part of the) body cavity, the muscle splits into two or three portions that separately pass through the bottom of the muscular apparatus suspending the cerebral
ganglion (Fig. 8.4a). The separate portions continue through the metasomal (part of
the) body cavity before they attach to the inner surface of the metasomal body wall
(Fig. 8.4a) (Herlyn 2017; Herlyn and Taraschewski 2017 and references therein).
Eversion and inversion of the presoma are usually repeated until the worm is
attached to the intestinal mucosa or deeper (Hammond 1966; Aguiar et al. 2018).
Characteristic for the resting position is a subsequent slight withdrawal of the neck
by contraction of the neck retractor, which also has a mesh-like organization
(Figs. 8.4a and 8.6c, d). As some of its strands enclose the lemniscs, usually paired
processes of the presomal tegument extending into the metasomal (part of the) body
cavity (Figs. 8.4 and 8.6d), contraction of the neck retractor presumably presses fluid
from the lemniscs into the presomal tegument, thus stiffening the everted proboscis.
Thereby, the transportation of fluid takes place via a so-called lacunar system
extending through tegument and lemniscs (e.g., Fig. 8.6c; Hammond 1966; Herlyn
2002, 2017; Herlyn and Taraschewski 2017). This may be the case or not, but it is
almost certain that there is no connection between the lacunar system inside the
tegument and lemniscs and any sub-tegumental structures including the body wall
musculature (compare Nielsen 2012). In addition, the musculature is not hollow,
although it may occasionally appear hollow due to preparation artifacts (for a discussion, see Herlyn and Taraschewski 2017; see also Nikishin 2004).
From a paleoparasitological point of view, the practical value of the above details
on the functional morphology of acanthocephalan anchoring is currently limited. In
principle, however, musculature and other decay-prone structures can fossilize
under certain conditions (Parry et al. 2018). In addition, most of the aforementioned
muscles are visible in total preparations of acanthocephalans (e.g., Fig. 5a, b in
Herlyn and Taraschewski 2017). Accordingly, the one or other of the aformentioned muscles might shine through the body wall of the yet to be discovered fossilized thorny-headed worm, comparable to the presumed alimentary tract in I. fellatus
(see Fig. 2a in Cong et al. 2017). Thus, if the fossil of a suspected endoparasite ever
discloses details of its internal organization, then a broader longitudinal strand
extending through the anterior body section (presoma) could correspond to the proboscis retractor of an acanthocephalan. Furthermore, strands with smaller diameter
that extend through the foretrunk (anterior portion of metasoma) could represent the
8 Thorny-Headed Worms (Acanthocephala): Jaw-Less Members of Jaw-Bearing…