Phylogenetics and Evolution
More on Acanthocephalans and Their Relatives
On muscles that are not hollow, a rete system that was not found, ligament sacs that are not paired, and an apical organ without sensory function
Based on morphological and ultrastructural investigations we were able to substantiate that jaws with tubular supportive rods represent an evolutionary novelty of a group comprising "rotifers" and gnathostomulids (see lh in below cross section through the pharynx of a gnathostomulid; A). Acanthocephalans also belong to this group, but have lost a digestive tract. Our data moreover suggest that the body wall musculature of acanthocephalans is not hollow (see below electron micrograph; B), though they might appear hollow in histological preparations. Furthermore, studying complete series of semithin sections from several acanthocephalan species we found no evidence for the existence of a rete system connecting body wall musculature and epidermal lacunar system.
Other investigations revealed that the "apical organ" in eoacanthocephalans is an epidermal intrusion (epidermal cone; see ec in below longitudinal section; A), but not a sensory organ as in archiacanthocephalans. Moreover, the apical sensory organs of archiacanthocephalans are composed of nerve endings and protrusions of a so-called support cell ("Stützzelle"; see ao in below longitudinal section; B). Thus, the structure of archiacanthocephalan apical sensory organs is very similar to that of lateral sensory organs (also lateral sense organs, lateral sensory pits, sensory pores, neck sensory organs) that are widely distributed within acanthocephalans (see avn and ct in below cross section; C). Mapping the character distribution on the acanthocephalan phylogeny suggests that apical sensory organs got lost at least once in acanthocephalans. Moreover, the epidermal cone most probably evolved as an evolutionary novelty in the stem lineage of Eoacanthocephala or a group comprising Eoacanthocephala and Polyacanthocephala (see publications: Weber et al. 2013).
Additional studies focused on the acanthocephalan trunk of these dioecious animals. The female genital tract contains a so-called uterine bell that functions as an egg-sorting apparatus: while mature eggs can pass, the uterine bell withholds immature eggs. We investigated the uterine bell at the cellular level and inferred a hyphethesis regarding its composition in the acanthocephalan basal pattern (= ground pattern; see shadings in below drawings; A-C). The data additionally showed that females of Paratenuisentius ambiguus (Eoacanthocephala) have only a single ligament sac (see li in below figure; D+E). The remainder trunk lumen has no own lining and hence represents a derivative of the primary body cavity.
More on the Molecular Evolution of Zonadhesin