Dracoderes abei, Higgins and Shirayama, 1990

Herranz, Maria, Sørensen, Martin V., Park, Taeseo, Leander, Brian S. & Worsaae, Katrine, 2020, Insights into mud dragon morphology (Kinorhyncha, Allomalorhagida): myoanatomy and neuroanatomy of Dracoderes abei and Pycnophyes ilyocryptus, Organisms Diversity & Evolution (New York, N. Y.) 20 (3), pp. 467-493 : 469

Myoanatomy of D. abei

Introvert

Myoanatomy of the introvert in D. abei includes ten primary spinoscalid muscles, ten sets of W-shaped spinoscalid retractors, ten pairs of long introvert retractors, three introvert circular muscles, and about twelve short introvert retractors (Figs. 2 and 3). The primary spinoscalid muscles (psm) are short and thick, situated inside the basal sheath of the primary spinoscalids attaching proximally in the dorsal region of the sheath and distally in the joint with the spinoscalid end piece (Figs. 2a, b and 3). W-shaped spinoscalid retractors (sr) alternate positions with the primary spinoscalids attaching to the introvert cuticle anteriorly at the level of the second spinoscalid ring and posteriorly at the level of the fourth spinoscalid ring (Figs. 2a–c and 3). When the W-shaped muscles (sr) contract and withdraw the head into the trunk, they invert and become M-shaped (Fig. 2c). The introvert long retractors (ilr) attach to the introvert cuticle at the level of the second row of spinoscalids and extend toward the trunk where they attach in dorsolateral and ventromedial positions at the pachycycli (cuticular thickenings of the anterior segment margins) of segments 3–8 (Figs. 2a, b, e, h and 3). The introvert circular muscles are thin (composed of one to three muscle fibers each), and their position changes depending on the level of introvert eversion. When the introvert is everted, the introvert circular muscles are situated underneath the most posterior spinoscalids rings (Figs. 2a, b and 3) and below the level of the neck when the introvert is fully retracted. The introvert short retractors (isr) attach anteriorly, adjacent to the introvert circular muscles, and posteriorly to the cuticle of segment 1 in a medial position (Figs. 2a, b and 3a–c). These muscles reach their maximum length when the introvert is extended (Fig. 2c).