The proboscis in living individuals is tongue-shaped with the ability of dorsoventral flattening. It can dorsally be depressed into a longitudinal deep groove (Figs. 1, 8, 9). The groove is successively widening on the posterior side of the proboscis and leads to the flattening of its ventral side. The proboscis is 2–2.5 times longer than the length of the collar. Proboscis elongated, slightly conical or even round in anaesthetized and fixed individuals (Figs. 3, 6, 7, 10). A basal sheath exists posterior to the proboscis stalk.
The collar is broader than long. Its anterior rim in living individuals is smooth or just very slightly curled (Figs. 1, 2). The collar has a slight forward inclination seen in actively creeping individuals (Fig. 1) as well as in some of the fixed ones in lateral view (Figs. 8, 10). The collar groove is a conspicuous transversal depression around the collar (Figs. 11, 12). The posterior part of the operculum is smooth and often covers the anteriormost gill opening.
The gill region is about two times as long as the proboscis length and has 9–17 dorsolateral pairs of elongated transversal gill openings. However, the individual on Fig. 2 could possibly have a few more pairs of gill openings, but this has been impossible to check (as well as the exact size of this individual). A median dorsal ridge exists along the entire trunk (Figs. 1, 5, 7). Lateral gonads are located in the posterior third to half of the branchial region and posteriorly (Figs. 2, 8, 9, 17). There is a tendency towards genital dorsolateral ridges along the trunk, but they are not very prominent.
Size Protoglossus bocki
It is smaller than any other species of acorn worm. No specimens are complete, so it is impossible to give the total length.
The thickest part is the collar region that can reach 1 mm in diameter. The longest part of an individual is less than 1 cm, and such pieces contain not only the gill, but also the genital region. The total length can therefore be estimated to about one and a half centimetre.
Colour of living individuals
The colour of the proboscis is uniformly light pink to golden yellow (Figs. 1, 2). The anterior rim of the collar is whitish (Figs. 1, 2, 3, 4, 5, 6, 7, 8, 9). Two slightly darker yellow transversal bands exist around the collar; the posterior of them coincides with the narrow circular depression around the collar, the so-called collar groove. The posterior part of the collar, the operculum, is whitish (Figs. 1, 2, 3, 4, 5, 6, 7, 8, 9). The trunk colour is uniformly light brownish yellow to golden yellow. The mucus gland tissue around the gill openings is slightly lighter yellow than the surrounding body wall. The intestine looks brown through the body wall (Fig. 1). The rose ovaries or white testes can be seen through the body wall (Figs. 2, 8, 9).
The proboscis is covered by a thick glandular and ciliated epithelium. Its musculature is well developed (Figs. 10, 11). The circular muscle fibre layer is thinner than the nerve fibre layer (Fig. 13). The nerve fibre layer of the proboscis is thickened middorsally (Fig. 13). There is a tendency towards a radial arrangement of the fibres in the longitudinal muscle layer. It is stronger developed laterally than dorsally, and its internal distribution seen in cross section coincides with the shape of the dorsal groove in creeping individuals (Figs. 1, 13).
The new species has a fairly wide protocoel around the heart and axial complex (Fig. 13). It extends to the anterior part of the proboscis and has a well-developed ventral mesenterium (septum). There is no dorsal mesenterium as the pericardium is attached directly towards the dorsal nerve thickening. The protocoel is curved towards the single external opening on the left side.
The axial complex (Figs. 10, 11, 12, 13) consists of a large pericardium dorsally attached under the dorsal nerve thickening. It covers the cardiac vesicle, also called heart or blood sinus, which, in turn, is supported by the stomochord, which has a longitudinal lumen along the centre of its axis. The stomochord lacks ventral caecum and extends along the axial complex (Fig. 12). The glomerulus consists of a folded system of vessels, but its organization is simple because of the small size (Fig. 13).
The proboscis skeleton is well developed in its anterior part. It has dorsal and lateral wing-like projections, clearly visible in cross section (Fig. 14), that support the stomochord and axial complex behind the glomerulus. Its two crura extend posteriorly into the collar and slightly curve and taper (Fig. 10). Two coelomic cavities fill the proboscis neck.
Peripharyngeal cavities are lacking, and the perihaemal coelom cavities extend only slightly into the collar region. The collar has a large coelomic cavity (mesocoel), completely divided by well-developed dorsal and ventral septa. The coelomopores from the mesocoel are large and open in the anterior, reduced gill chambers. Two peribuccal coelome diverticules are present.
The crura of the proboscis skeleton extend to the posterior part of the collar and partly surround the mouth cavity. They project slightly laterally and ventrally. The proboscis skeleton retractor musculature forms lateral longitudinal muscle bands (Fig. 10) that are strong, but the other musculature is weakly developed. The collar region is completely separated from the body coelom (metacoel) by a strong septum. A neuropore could not be observed, but lacunae in the dorsal nerve cord exist.
The trunk region has strongly developed ventral longitudinal muscles, which extend far upwards along the sides of the body (Figs. 15, 16). The contraction of this musculature is usually causing a ventral shrinking of the animals during fixation. There is a pair of small dorsal muscle bands and prominent dorsal and ventral nerve chords (Figs. 15, 16).
The gill intestine, at least in its middle parts, considerably exceeds the hepatic intestine in extent (Fig. 12). The gill pockets are simple and well developed. They are up to 400 μm in length, which is half of the body thickness (Fig. 15, 16) and consists of 9–17 pairs. The gill skeleton is weakly developed, but at least, the branchial bars are prominent (Fig. 16). Large numbers of mucus producing glands surround the outer gill openings (Figs. 1, 16). They are elongated, placed transversely, and generally built up by large, sack-shaped cell bodies.
The anterior parts of the gonads extend into the gill region. The ovaries overlap only the posterior part (Fig. 8), but the testes may overlap more than half of the gill region (Figs. 2, 9). The gonads are dorsolateral and up to 400 μm in diameter and contain several yolk-rich oocytes, up to 250 μm in diameter (Fig. 17).
Sixten Bock noted that P. bocki has a characteristic “medicine smell”, and Burdon-Jones (1956) wrote that P. koehleri has a smell resembling iodoform, a kind of halogenated disinfectant used at his time. Similarly, smelling bromophenolic or haloorganic compounds have been found in P. graveolens (Giray and King 1996, 1997) and P. mackiei (Deland et al. 2010).