The Stylet

Diversity and Systematics of Acoela and Nemertodermatida

Family level classification of Acoela

Family level classification  of Acoela


The following is an overview of the diagnoses of the families of of Acoela after some synonymizations proposed in U. Jondelius, A. Wallberg, M. Hooge, and O. I. Raikova, “How the Worm Got its Pharynx: Phylogeny, Classification and Bayesian assessment of Character Evolution in Acoela,” Systematic Biology, 2011.doi:10.1093/sysbio/syr073

Proporidae, Graff 1882

After the synonymization of Haploposthiidae, Westblad 1948  with Proporidae, this is a morphologically heterogenous group that comprises i) Acoela with long a ciliated pharynx tube that opens frontally or anteroventrally. Male copulatory organ opens terminally or subterminally at posterior end, or in some cases ventrally. Vesicula seminalis present; penis may be absent. Antrum masculinum either long and tubiform or inconspicuous; ii)Acoels with very simple copulatory organs. At most the copulatory organ consists of a tube-shaped male antrum and a spherical seminal vesicle. Either the antrum or the seminal vesicle or both can be absent. Penis is lacking in may species [Haploposthia albiventer, H. rubra and H. lactomaculata have a very small penis papilla]. Male gonopore terminal, subterminal, or ventral. The recently synonymized Polycanthidae also has a ciliated male antrum and a posteroterminal male pore, but possesses a multi-layered seminal vesicle composed of five layers of alternating longitudinal and circular muscles:outer-longitudinal muscles, outer-circular muscles, middle-longitudinal muscles, inner-circular muscles, and inner-longitudinal muscles.

Convolutidae, Graff 1905

Body-wall musculature with circular, longitudinal, and longitudinal crossover muscle fibers in both the dorsal and ventral body wall, and U-shaped fibres in the ventral body wall. Male copulatory organ with muscular, often glandular, tubular penis. Penis musculature with inner circular fibers and outer longitudinal fibers that anastomose or cross over each other. When present, a muscular seminal vesicle surrounds all of, or a portion of the penis.Male gonoporeventral, subterminal, or terminal at posterior end; usually positioned behind opening to female seminalbursa.Spermatozoa with 9+0 axonemes and axial microtubules.Algal symbionts often present. Ocelli, when present, contain reflective platelets in the pigment cell and lack cilia or microvilli as receptororganelles. Sagittiferidae, recently synionymized with Convolutidae have a saccate male antrum, and some species have sagittocysts.

Isodiametridae, Hooge and Tyler, 2005

Acoels with a ventral mouth opening.Body-wall musculature with circular,longitudinal, and longitudinal crossover muscle fibers in both the dorsal and ventral body wall, and U-shapedfibers in the ventral body wall.Male copulatory organ with muscular, isodiametric, tubular penis, oftenvery glandular.Penis musculature with inner circular and outer non-anastomosing longitudinal fibers. Penisis invaginated into a muscular seminal vesicle, if present.Male gonopore ventral, subterminal,supraterminal, or terminal at posterior end.Spermatozoa with 9+2 axonemes and cortical microtubules.Neverwith symbiotic algae. Ocelli, when present, do not contain platelets in the pigment cell.

Diopisthoporidae Westblad, 1940

Male genital pore and mouth opening at posterior end. A muscular, ciliated pharynx tube present. Germinal tissue compact. Ovary and testis unpaired. Single genus is Diopisthoporus. Frontal organ present. Unpigmented. No “rhabdites”. Female accessory organs absent.


Hallangiidae, Westblad 1948

Mouth and pharynx at front end as in Proporus. Pharynx simplex with ciliated, insunk epithelium and strong musculature. Frontal organ exceptionally strongly developed. Ovary paired or weakly dichotomous. Testis paired. Two genital pores. Bursa seminalis without mouthpiece. Copulatory organ consisting of a vesicula seminalis and penis.



Otocelididae, Westblad 1948

With seminal bursa and more or less long vagina. Vagina always behind penis, either opening with it or through its own pore. Penis invaginated into vesicula seminalis in most species. Eyes can be lacking. Bursal nozzle can be absent.


Hofsteniidae, Dörjes 1968

Acoela with epithelial nervous system or with longitudinal nerves lying to the inside of the body-wall musculature. Mouth opening ventral, near the front end or terminal. Pharynx very long and tubiform, with simple or powerful musculature, or bell-shaped. Testis diffuse. Vesicula seminalis can be lacking. Vesicula granulorum with penis, which consists of sclerotized needles. Male genital pore immediately behind the mouth opening, widening into an antrum masculinum. Ovaries ventral, either paired or follicular. Female accessory organs lacking. 


Paratomellidae, Dörjes 1966

Germ tissue diffuse. Male germ zone limited to the sides of the first third of the body. Egg-forming tissue laterally, reaching far behind the male complex. No further organs in female system.


Nadinidae, Dörjes 1968

Mouth opening ventral, with large archipharynx. Pharynx ciliated outer portion short, unciliated inner portion long and with circular and longitudinal muscles. Genital pore ventral. Vagina opens anterior to penis (two genital openings). Bursa present. paired ovary. Diffuse testis.


Anaperidae, Dörjes 1968

Genital pore terminal or subterminal. Seminal vesicle absent as a rule; penis not invaginated into seminal vesicle. Male antrum well-developed, always with many sclerotized stimulatory organs.

Antigonariidae Dörjes. 1968

Mouth opening ventral in anterior end. Male genital pore shortly behind mouth. An unpaired germinal zone in mid body. Sperm development toward anterior, egg development toward posterior. Ovary and testis unpaired. Male copulatory organ a long ciliated antrum tube and a false seminal vesicle. Seminal vesicle and penis lacking. Female accessory organs absent.



Actinoposthiidae, Hooge 2001

Copulatory organ with penis stylets, built of either sclerotized or muscular elements,never invaginated into seminal vesicle. Seminal vesicle may be absent. Male genital pore variable in position.Mouth opening ventral.


Mecynostomidae, Dörjes 1968

Male copulatory organ either a globular structure inside of which is a proximal seminalvesicle and a distal glandular region which serves as a penis or well-developed single, paired, or multiple copulatory organs built of tightly packed or fused sclerotized stylet needles (stylet absent in Childia dubium). Proximal part of the stylet inserted into seminal vesicle (if present). Body often dorsoventrally flattened. Species within the genus Childia have body-wall musculature with longitudinal fibers positioned outside circular fibers. Ventral diagonal muscles absent. Ventral straight longitudinal muscles between frontal pore and mouth present. Dorsal diagonal muscles present. Mouth opening ventral. Sperm with 9 + 1 axonemes.


Solenofilomorphidae, Dörjes 1968

Acoela with unpaired dorsal ovaries and testes. Germ tissue mixed. Copulatory organ in mid-body. Ovary reaching far behind male complex. Mouth opening subterminal at front end connects proximally to ciliated tubiform pharynx simplex.


Dakuidae, Hooge 2003

Male copulatory organ a glandular penis surrounded by seminal vesicle with thick muscles in well-defined longitudinal and circular orientations. Conical glandular penis composed of granular lamellae-like cells surrounding a lumen that opens to subterminal male gonopore. Seminal vesicle with thinstrap-like outer-longitudinal muscles, and thick (~7 µm) inner-circular fibers. Ventral body-wall musculature with straight longitudinal muscles between frontal pore and mouth, and lacking anterior ventral diagonal muscles positioned between outer circular and inner longitudinal muscles.

Taurididae, Kostenko 1989

Male copulatory apparatus characterized by consecutive connection of the vesicula seminalis, vesicula granulorum and ciliated antrum. “Morphological investigation of the male and female copulatory organs made it possible to compare them with genital organs of other families and to make the conclustion that a new family must be established for this species” (Kostenko 1989). 


Anthroposthidae, Faubel 1976

Ciliated male atrium inserted into a penis sheath or sac as a direct inpocketing of the epidermis. Male antrum adjoins proximally a vesicula granulorum. Proximally an unciliated granular vesicle closes off the atrium directly. Seminal vesicle or false seminal vesicle present. Male gonopore ventral or terminal at posterior end. Without penis.

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith