Heart, pericardium, coelomoduct openings, and juvenile gonad in Chaetoderma nitidulum and Falcidens caudatus (Mollusca, Aplacophora)

Summary

• 1. The single auricle of Chaetoderma nitidulum originates as double invaginations of pericardial epithelium; these invaginations lie laterally at the base of an intrapericardial channel and receive blood from a capacious gill sinus into which empty the dorsally situated efferent vessels of the gills. The ventricle and aortal bulb with its strong sphincter change shape markedly during systole and diastole. The auriculo-ventricular valve is a cup-shaped check valve, the blind end facing posteriorly toward the auricle. It is attached medially both dorsally and ventrally to the wall of the auriculo-ventricular opening, thus forming a double passage. The heart of Falcidens remains to be described in detail. The ventricle in neither genus is open to the dorsal sinus.

• 2. The pericardium in Falcidens caudatus and Chaetoderma nitidulum divides medially around the dorsal gill retractors, forming an intrapericardial channel connecting the gill sinus and dorsal sinus. The pericardium is completely closed off from the dorsal sinus. A pair of lateral V-shaped extensions open ventrally into the coelomoducts.

• 3. The male coelomoduct openings into the mantle cavity of C. nitidulum and F. caudatus pass through strong sphincters, which may or may not be protruded into papillae. The female openings are slits also surrounded by sphincters. Paired funnel-shaped areas of the mantle wall epithelium of females only are produced into columnar gland cells and surround the openings; their function is not known.

• 4. The gonad primordium of juvenile C. nitidulum is paired; the anlagen lie in paired coelomic lumina. As in Bivalvia, adults do not distinctly show a paired condition in the ripe gonad.

• 5. The shape of the heart, the supposed number of auriculo-ventricular openings, and the type of coelomoduct openings into the mantle cavity are not useful taxonomic characters for species differentiation in the family Chaetodermatidae.