Microcosmus helleri

Solitary or aggregated the specimens have a wrinkled tunic covered with sand and epibionts (Fig. 8 A, B). The siphons are well spaced with papillae. The internal side of the tunic on the body is purple. The reflex tunic inside both siphons, at its upper part, has brownish red longitudinal lines and no spinules. More internally at the oral siphon are 4 thin round lobes protruding into the siphon cavity (Fig. 9 B). The internal tunic lining tightly adheres to a thin velum anterior to the bushy oral tentacles. The body wall has a strong musculature in crossed bundles (Fig. 8 C). The prepharyngeal band is double and curved dorsally to enclose a dorsal tubercle more or less protruding with inrolled horns (Fig. 9 C). The branchial sac has 6 high folds per side (Fig. 9 A). The dorsal lamina has a plain edge. The transverse vessels are irregular and some of them particularly large are linked on their length to the body wall, gonads and gut reducing the space of the atrial cavity. The branchial formula on the right side of a specimen 3.5 cm large is: E- 2 (17) 4 (18) 4 (21) 4 (22) 4 (20) 4 (22) 2 - DL The longitudinal vessels unite at their base without papillae. The gut and gonads are included into the body wall except the short gonoducts and the anus. The digestive tract forms a long closed loop along the endostyle (Fig. 8 C). The stomach is covered by a large papillated digestive gland (Fig. 8 C). The intestine is narrow and the anus has 2 lobes. One lobe of the left gonad lies partly into the anterior part of the gut loop, 2 other lobes, not distinctly separated, are placed along the descending limb of the intestine (Fig. 8 C). The right gonad in 2 or 3 close lobes is placed ventrally (Fig. 8 C). The testis vesicles form a ramified tree lying between the ovary and the body wall and can be seen in transparency from the external side of the body wall. There is a ring of thin thread-like papillae at the base of the thin atrial velum which adheres to a thickened part of the tunic lining (Fig. 8 C). M. helleri is a widely distributed species, characterised among the Microcosmus having a sandy tunic, 6 branchial folds per side, absence of siphonal spinules and 4 large lobes at the inner base of the oral siphon, anterior to the oral tentacles.