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chance of transfer into a definitive host (e.g., Kennedy 2006). Taken together, every
jawed vertebrate that at least occasionally feeds on mandibulates appears to be a
potential host in the one way or another. Correspondingly, acanthocephalan diversity might be much higher than suggested by the figure of about 1,300 species currently known (Gibson et al. 2014). In support of this expectation, ongoing
descriptions of species from previously under-investigated gnathostome vertebrates let the count of acanthocephalan species continuously grow (e.g., Gomes
et al. 2015; Gautam et al. 2019).
Inside the alimentary tract of their definitive hosts, thorny-headed worms grow to
body sizes of mostly several millimeters up to centimeters (Fig. 8.3), while only few
of the extant species exceed body sizes of 10 cm or more. Whether a tiny creature
or a considerable worm, males generally remain smaller than females (see, e.g.,
Petrochenko 1956, 1958). However, except for this inverse sexual dimorphism in
body size, male and female acanthocephalans have pretty much the same outer
appearance: The anterior-most section is occupied by the holdfast organ (proboscis)
followed by the neck. Proboscis and neck together constitute a retractable
Fig. 8.3 Drawings of the fish-parasite Neoechinorhynchus mexicoensis (Eoacanthocephala). (a)
Adult female with ovarian balls and eggs floating inside the metasomal body cavity. (b) Adult male
with syncytial cement gland and two tandem-arranged testes. Except for the proboscis-receiving
apparatus, which in eoacanthocephalans is comprised of receptacle and receptacle protrusor, the
musculature is not shown (Modified after Pinacho-Pinacho et al. 2014)
H. Herlyn
chance of transfer into a definitive host (e.g., Kennedy 2006). Taken together, every
jawed vertebrate that at least occasionally feeds on mandibulates appears to be a
potential host in the one way or another. Correspondingly, acanthocephalan diversity might be much higher than suggested by the figure of about 1,300 species currently known (Gibson et al. 2014). In support of this expectation, ongoing
descriptions of species from previously under-investigated gnathostome vertebrates let the count of acanthocephalan species continuously grow (e.g., Gomes
et al. 2015; Gautam et al. 2019).
Inside the alimentary tract of their definitive hosts, thorny-headed worms grow to
body sizes of mostly several millimeters up to centimeters (Fig. 8.3), while only few
of the extant species exceed body sizes of 10 cm or more. Whether a tiny creature
or a considerable worm, males generally remain smaller than females (see, e.g.,
Petrochenko 1956, 1958). However, except for this inverse sexual dimorphism in
body size, male and female acanthocephalans have pretty much the same outer
appearance: The anterior-most section is occupied by the holdfast organ (proboscis)
followed by the neck. Proboscis and neck together constitute a retractable
Fig. 8.3 Drawings of the fish-parasite Neoechinorhynchus mexicoensis (Eoacanthocephala). (a)
Adult female with ovarian balls and eggs floating inside the metasomal body cavity. (b) Adult male
with syncytial cement gland and two tandem-arranged testes. Except for the proboscis-receiving
apparatus, which in eoacanthocephalans is comprised of receptacle and receptacle protrusor, the
musculature is not shown (Modified after Pinacho-Pinacho et al. 2014)
H. Herlyn