The first pair of thoracopods (Fig. 7 A) to Thp 7 are all similar, whereas Thp 8 is vestigial. The following descriptions are based on a detailed examination of the right Thp 1 of three adult females of M. norƲegica. Cuticle and skeletal structures of thoracopods in M. norvegica The following description refers collectively to Thp 1 – 7, if not specified otherwise. The ventral cuticle of the first thoracomere is coalesced with that of the cephalon. The density of the cuticle of this cephalothoracic sternum appears higher than in the posterior thoracomeres. Anteriorly, the thoracic cuticle articulates with the maxilla, which is tightly fitted against the anterior wall of the coxa and basis of Thp 1. In all thoracomeres, the posterior thoracic cuticle is folded in an anterior direction, forming a broad roof-like apodeme (partly dashed curved arrow in Fig. 7 B, C) of moderate density that hangs over the posterior half of the proximal foramen of the leg (i. e. of the coxa). Medially, the cuticle of the thorax forms a posterolaterally and dorsally directed finger-like extension (dashed black outline on Th in Fig. 7 B). The protopod of M. norƲegica comprises a coxa (onethird) and a subdivided basis (two-thirds of the total length of the protopod). The coxa is predominantly made up of its more solid distal margin, which forms a half-ring reaching from posterior over medial to anterolateral (Co in Fig. 7 D), where it connects to a plate of less dense cuticle. From the anterolateral corner of this plate, a solid cuticular beam reaches dorsad (Cb in Fig. 7 C, D) and articulates with the thorax (* l in Fig. 7 B – D). Posterolateral on the coxa, an epipod arises, which on Thp 1 is plate-like (Ep in Fig. 7 A, B), extending somewhat dorsad, but mostly ventrad (almost reaching the exopod). On the posterior thoracopods, the epipod splits into two coiled branches (one dorsal and one ventral) that ramify into several smaller filaments; on Thp 6 – 8, the epipods show the highest complexity, with an additional ventral off-branch that ramifies in a similar way. Medially attached to the coxa of Thp 1, a setose endite is present (En, Fig. 7 A, C). The endite forms the medial articulation point with the thorax, immediately beneath the finger-like extension (* m in Fig. 7 B). On the following thoracopods, the endite is much smaller, if present at all (Fig. 7 E). Apart from the cuticular half-ring constituting the distal margin of the coxa, its anterior and posterior parts are mostly membranous, with two (posterior and posterolateral) invaginations. The posterolateral invagination forms a tight proximodistal fold, which expands into the central space of the coxa (curved arrows in Fig. 7 D), where it forms a solid and highly complex endoskeletal structure (dark blue part of Co in Fig. 7 B – D; see also the Supporting Information, Video S 1). Starting from a posterolateral arm, the structure reaches into the centre of the coxa, where it expands into the dorsal and ventral directions. Dorsally, it reaches onto the roof-like apodeme of the thorax. Ventrally, it reaches onto the proximal margin of the basis by extending another two straight arms in anterior and posterior directions. Table 2. Continued The basis comprises a solid distal part (Ba in Fig. 7 A – E), whereas its proximal part (InB in Fig. 7 C – E) consists of three posterior and lateral plates and a broad anterior part. Owing to the resemblance to the anaspidan intrabasis, the proximal part (s) of the basis are referred to as the intrabasis in the following description. The anterior part of the intrabasis is separated from the larger distal basis by an inconspicuous furrow (dashed white line in Fig. 7 C, E). Anteriorly, it forms a small bulb at its proximal margin (Fig. 7 E), into which the anterior arm of the coxal endoskeleton reaches. Proximal to the anterior bulb, at the proximal margin of the intrabasis, sits a somewhat triangular plate (white outline on InB and partly dashed curved arrow in Fig. 7 C), which articulates with the coxa (a + in Fig. 7 C). Another articulation point is realized between the coxa and the posteromedial plate of the intrabasis (p + in Fig. 7 D), resulting overall in a bicondylar coxa – intrabasis articulation (ax Co-InB in Fig. 7 B). The medial cuticle of the distal basis appears robust, whereas laterally it is mostly membranous. In this area, two rigid, finger-like structures emerge (one posterolateral and another distoanterolateral) and articulate with two hook-like structures of the exopod (Fig. 7 B, C; see also Supporting Information, Video S 1). The posterolateral articulation is supported by a longish sclerite positioned immediately distal to the posterolateral finger. The medial cuticle of the distal basis appears robust and carries numerous setae. Distally, the basis is coalesced with the endopod (i. e. with the ischium), with a clear suture but no interpodomere membrane or articulation points (Fig. 7 C, D). Within the lateral membrane of the distal basis, the exopod attaches (Ex in Fig. 7 A – D). Its peduncle comprises a short posteroventral stem and a much longer distal portion, followed by the flagellum. The peduncle is anteroposteriorly flattened and has a prominent distodorsal pointed protrusion. Proximally, the peduncle articulates with the basis both anterolaterally and posterolaterally via hook-like structures (one from the peduncle stem and one from the distal portion) that are interlocked with the finger-like structures of the distal basis (short arrow in Fig. 7 B; see also Supporting Information, Video S 1). The endopod consists of the ischium, merus, carpus, propodus and dactylus. The ischium (Is in Figs 7 A, 8 A) is about twice as long as the protopod. Anteriorly, the ischium forms an articulation with the merus (arrow in Fig. 8 D). However, posteriorly an articulation point is not clearly identifiable. Nevertheless, the proximal margin of the merus forms a prominent posterolateral protrusion, which overlaps slightly with the distal margin of the ischium (dashed line in Fig. 8 D). The merus (Me in Fig. 8 A) appears slightly longer than the ischium. Two long sclerites / tendons emerge laterally and medially from the proximal margin of the merus and reach far into the ischium. A bicondylar articulation with the carpus is formed by a medial and a lateral articulation point, respectively (ax Me-Ca in Fig. 8 A). The carpus (Ca in Fig. 8 A) measures approximately one-third of the total length of the merus. One short and two long sclerites / tendons emerge anteriorly (long) and posteriorly (long and short) from the proximal margin of the carpus and reach far into the merus. The distal margin forms anteriorly a noselike condyle that articulates with a broad anterior depression at the proximal margin of the propodus (Fig. 8 B, C). A thin, longish sclerite articulates laterally with the distal margin of the carpus and posteriorly with the proximal margin of the propodus (Fig. 8 B, C). Combined, these structures probably realize a somewhat less restricted bicondylar articulation. the furrow between the intrabasis and basis; black lines emphasize individual plates of the coxa; white lines and partly dashed curved arrows emphasize the posterior thorax apodeme and the proximal anterior triangular plate of the intrabasis. D, posterior view; dashed white line emphasizes the posterior plates of the intrabasis; dashed white circle indicates the anterior bulb of the intrabasis; black lines emphasize individual parts of the coxa and the margins of the intrabasis; curved arrows indicate posterolateral coxa invagination. E, Thp 3, anterior view; dashed line emphasizes the furrow between intrabasis and basis. Abbreviations: a *, anterior articulation point; ant, anterior; ax, articulation axis between […]; Ba, basis; Ca, carpus; Cb, proximolateral cuticular beam of coxa; Co, coxa; Da, dactylus; dis, distal; En, endite; Ep, epipod; Ex, exopod; InB, intrabasis; Is, ischium; l * and * l, lateral articulation point; lat, lateral; * m, medial articulation point; Me, merus; med, medial; p *, posterior articulation point; Pr, propodus; prox, proximal; Th, thorax. The propodus (Pr in Fig. 8 A) is about twice as long as the carpus. One short and three long sclerites / tendons emerge from the anteromedial, posteromedial, anterolateral (all three long) and posterolateral (short) corners of the proximal margin of the propodus and reach into the carpus. The propodus articulates with the dactylus by a medial and a lateral articulation point, respectively (ax Pr-Da in Fig. 8 A). The dactylus is about half the length of the propodus in Thp 1 (Da in Figs 8 A, 9 E) and somewhat longer than half the propodus in the posterior thoracopods. Two long sclerites / tendons emerge anteriorly and posteriorly from the proximal margin of the dactylus and reach into the propodus. The podomere tapers from its proximal to its distal margin, except for Thp 2, where it is more lateromedially flattened, forming a slight distoposterior bulge before tapering towards a distoanterior tip.
