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receptacle retractor and/or neck retractor of an acanthocephalan. In addition, a baglike structure between a smaller (presomal) and a wider (metasomal) body cavity
could testify to the muscular apparatus that once suspended the cerebral ganglion of
a palaeacanthocephalan, eoacanthocephalan, gigantorhynchid, moniliformid, or
oligacanthorhynchid. But if such a septum is not visible in the hypothetical fossil
and the outer contour approximates a balloon with a worm-like appendix, the remain
could originate from an apororhynchid.
8.4.3 Presomal Sensory Organs
Whether involved in the eversion of the proboscis or not, the muscular apparatus
always carries the cerebral ganglion (Fig. 8.4a). Obviously, the acanthocephalan
cerebral ganglion should not only steer muscular activity but also integrate incoming signals. In some acanthocephalans, such information will come from comparably prominent sense or sensory organs at the base of the neck (lateral sensory
organs) and the proboscis apex (apical sensory organs). These organs reside just
beneath the bottom of circular pits shaped by tapering tegument, which resemble
pores (Gee 1987). Depending on the taxon, these pits can reside on the tips of conical elevations of the presomal tegument, comparable to the crater of a volcano
(Fig. 8.4c, d). While this may sound quite complicated, things are getting easier
with respect to the sensory organs themselves. In fact, lateral and apical sensory
organs are identical in their general organization: They are essentially terminal bulbous swellings of the processes of a so-called support cell (which is a syncytium)
into which dendritic endings are imbedded (Fig. 8.4b–d). Proximally, the dendritic
differentiations leave the bulbs and unite to nerves that extend to the cerebral ganglion (e.g., Harada 1931; Gee 1987; Herlyn et al. 2001).
Although their occurrence is widespread, these sensory organs are not present
in all extant acanthocephalan species. According to the available data, extant
acanthocephalans possess either two apical sensory organs in addition to a pair of
lateral sensory organs (Archiacanthocephala: Gigantorhynchida, Moniliformida)
or one apical organ plus two lateral sensory organs (Archiacanthocephala:
Oligacanthorhynchida) or two lateral sensory organs only (probably all eoacanthocephalans and some palaeacanthocephalans). The last alternative is the absence
of any sensory organs of the described type (other palaeacanthocephalans) (e.g.,
Gee 1987; survey in Herlyn et al. 2001). How apororhynchids fit into the picture
remains to be elucidated. Nonetheless, the available data suggest that a duplication of the presomal sensory apparatus occurred in archiacanthocephalans, namely
from a state of only two lateral sensory organs (Fig. 8.4b) to the formation of two
lateral plus two apical sensory organs (Fig. 8.4c). Once established, both apical
sensory organs seem to have fused to an unpaired structure in the stem line of
crown- Oligacanthorhynchida (Fig. 8.4d; Weber et al. 2013).
Here, too, the practical value of this morphological description can only arise if
a fossil endoparasite with corresponding soft tissue preservation will ever be discovered. Nonetheless, the fossilization of decay-prone structures such as the
H. Herlyn
receptacle retractor and/or neck retractor of an acanthocephalan. In addition, a baglike structure between a smaller (presomal) and a wider (metasomal) body cavity
could testify to the muscular apparatus that once suspended the cerebral ganglion of
a palaeacanthocephalan, eoacanthocephalan, gigantorhynchid, moniliformid, or
oligacanthorhynchid. But if such a septum is not visible in the hypothetical fossil
and the outer contour approximates a balloon with a worm-like appendix, the remain
could originate from an apororhynchid.
8.4.3 Presomal Sensory Organs
Whether involved in the eversion of the proboscis or not, the muscular apparatus
always carries the cerebral ganglion (Fig. 8.4a). Obviously, the acanthocephalan
cerebral ganglion should not only steer muscular activity but also integrate incoming signals. In some acanthocephalans, such information will come from comparably prominent sense or sensory organs at the base of the neck (lateral sensory
organs) and the proboscis apex (apical sensory organs). These organs reside just
beneath the bottom of circular pits shaped by tapering tegument, which resemble
pores (Gee 1987). Depending on the taxon, these pits can reside on the tips of conical elevations of the presomal tegument, comparable to the crater of a volcano
(Fig. 8.4c, d). While this may sound quite complicated, things are getting easier
with respect to the sensory organs themselves. In fact, lateral and apical sensory
organs are identical in their general organization: They are essentially terminal bulbous swellings of the processes of a so-called support cell (which is a syncytium)
into which dendritic endings are imbedded (Fig. 8.4b–d). Proximally, the dendritic
differentiations leave the bulbs and unite to nerves that extend to the cerebral ganglion (e.g., Harada 1931; Gee 1987; Herlyn et al. 2001).
Although their occurrence is widespread, these sensory organs are not present
in all extant acanthocephalan species. According to the available data, extant
acanthocephalans possess either two apical sensory organs in addition to a pair of
lateral sensory organs (Archiacanthocephala: Gigantorhynchida, Moniliformida)
or one apical organ plus two lateral sensory organs (Archiacanthocephala:
Oligacanthorhynchida) or two lateral sensory organs only (probably all eoacanthocephalans and some palaeacanthocephalans). The last alternative is the absence
of any sensory organs of the described type (other palaeacanthocephalans) (e.g.,
Gee 1987; survey in Herlyn et al. 2001). How apororhynchids fit into the picture
remains to be elucidated. Nonetheless, the available data suggest that a duplication of the presomal sensory apparatus occurred in archiacanthocephalans, namely
from a state of only two lateral sensory organs (Fig. 8.4b) to the formation of two
lateral plus two apical sensory organs (Fig. 8.4c). Once established, both apical
sensory organs seem to have fused to an unpaired structure in the stem line of
crown- Oligacanthorhynchida (Fig. 8.4d; Weber et al. 2013).
Here, too, the practical value of this morphological description can only arise if
a fossil endoparasite with corresponding soft tissue preservation will ever be discovered. Nonetheless, the fossilization of decay-prone structures such as the
H. Herlyn