290
Gazi et al. 2016). However, if the contour of the hypothetical fossil is more reminiscent of a balloon with a shorter worm-like appendix, it could originate from an
archiacanthocephalan, particularly a member of Apororhynchida (Herlyn 2017).
8.4.2 Presomal Musculature and Anchoring
Internally, the transition from presoma to metasoma is marked by a muscular apparatus which suspends the cerebral ganglion or brain and probably consists of two
muscular layers in all extant species. The only complication in this general scheme
is that each layer can be either a continuous muscular sheath or a muscular mesh of
anastomosing strands. In Palaeacanthocephala, both muscular layers are continuous
and thereby have such intimate contact that the entire apparatus has a double-walled
appearance (Fig. 8.4a). Archiacanthocephalans belonging to Moniliformida show
the same pattern. In archiacanthocephalans belonging to Gigantorhynchida and
Oligacanthorhynchida as well as in all members of Eoacanthocephala, only the
inner layer is a sheath-like muscle, whereas the outer layer forms a loose muscular
mesh (Figs. 8.4b–d). Accordingly, the muscular apparatus suspending the cerebral
ganglion looks single-walled in these species (Herlyn 2002; Herlyn and Taraschewski
2017). The same pattern seems to be realized in Polyacanthocephala (Amin 1987).
Yet another way in which both muscle layers can be arranged is shown by the archiacanthocephalan taxon Apororhynchida: here both muscle layers consist of delicate
vela-like muscular strands (Herlyn 2017).
Unfortunately, the established terminology does not reflect the homology of the
two internal muscular layers at the presoma-metasoma transition. Thus, it is common practice to refer to a double-walled receptacle, when both muscular layers are
sheath-like and have intimate contact (Palaeacanthocephala, Moniliformida). The
layers themselves are then considered as inner and outer wall of the receptacle
(Fig. 8.4a). Yet, the receptacle is commonly regarded as single-walled, when only
the inner layer is sheath-like (Eoacanthocephala inclusively Polyacanthocephala,
Gigantorhynchida, Oligacanthocephala). Then, this inner layer is termed just receptacle, while the established name for the strands of the receptacle-surrounding muscle is receptacle protrusor (Figs. 8.4b–d and 8.6c, d; Herlyn and Taraschewski 2017;
also Amin 1987). The use of alternative names for the same muscles further complicates matters (for a survey, see Herlyn and Taraschewski 2017).
The alternative organization of the muscular apparatus carrying the cerebral ganglion has functional consequences: When both muscular layers are mesh-like
(Apororhynchida), their contraction should not increase the pressure inside the presomal part of the body cavity relative to its metasomal portion (Herlyn 2017).
However, as long as at least one of the muscular layers has a sheath-like organization
(all other taxa of extant Acanthocephala; Figs. 8.4, 8.5a and 8.6c, d), presomal and
metasomal body cavities are separated. Then, contraction of the either single- or
double-walled muscular apparatus increases the hydrostatic pressure inside the presomal body cavity. This again leads to the eversion of neck and proboscis, which can
H. Herlyn
Gazi et al. 2016). However, if the contour of the hypothetical fossil is more reminiscent of a balloon with a shorter worm-like appendix, it could originate from an
archiacanthocephalan, particularly a member of Apororhynchida (Herlyn 2017).
8.4.2 Presomal Musculature and Anchoring
Internally, the transition from presoma to metasoma is marked by a muscular apparatus which suspends the cerebral ganglion or brain and probably consists of two
muscular layers in all extant species. The only complication in this general scheme
is that each layer can be either a continuous muscular sheath or a muscular mesh of
anastomosing strands. In Palaeacanthocephala, both muscular layers are continuous
and thereby have such intimate contact that the entire apparatus has a double-walled
appearance (Fig. 8.4a). Archiacanthocephalans belonging to Moniliformida show
the same pattern. In archiacanthocephalans belonging to Gigantorhynchida and
Oligacanthorhynchida as well as in all members of Eoacanthocephala, only the
inner layer is a sheath-like muscle, whereas the outer layer forms a loose muscular
mesh (Figs. 8.4b–d). Accordingly, the muscular apparatus suspending the cerebral
ganglion looks single-walled in these species (Herlyn 2002; Herlyn and Taraschewski
2017). The same pattern seems to be realized in Polyacanthocephala (Amin 1987).
Yet another way in which both muscle layers can be arranged is shown by the archiacanthocephalan taxon Apororhynchida: here both muscle layers consist of delicate
vela-like muscular strands (Herlyn 2017).
Unfortunately, the established terminology does not reflect the homology of the
two internal muscular layers at the presoma-metasoma transition. Thus, it is common practice to refer to a double-walled receptacle, when both muscular layers are
sheath-like and have intimate contact (Palaeacanthocephala, Moniliformida). The
layers themselves are then considered as inner and outer wall of the receptacle
(Fig. 8.4a). Yet, the receptacle is commonly regarded as single-walled, when only
the inner layer is sheath-like (Eoacanthocephala inclusively Polyacanthocephala,
Gigantorhynchida, Oligacanthocephala). Then, this inner layer is termed just receptacle, while the established name for the strands of the receptacle-surrounding muscle is receptacle protrusor (Figs. 8.4b–d and 8.6c, d; Herlyn and Taraschewski 2017;
also Amin 1987). The use of alternative names for the same muscles further complicates matters (for a survey, see Herlyn and Taraschewski 2017).
The alternative organization of the muscular apparatus carrying the cerebral ganglion has functional consequences: When both muscular layers are mesh-like
(Apororhynchida), their contraction should not increase the pressure inside the presomal part of the body cavity relative to its metasomal portion (Herlyn 2017).
However, as long as at least one of the muscular layers has a sheath-like organization
(all other taxa of extant Acanthocephala; Figs. 8.4, 8.5a and 8.6c, d), presomal and
metasomal body cavities are separated. Then, contraction of the either single- or
double-walled muscular apparatus increases the hydrostatic pressure inside the presomal body cavity. This again leads to the eversion of neck and proboscis, which can
H. Herlyn