First record on the development of the larva of the basally branching nemertean species Carinina ochracea (Palaeonemertea)

Nemertea are a clade of unsegmented, vermiform, mostly marine Spiralia. While it is generally accepted that Nemertea develop via a pelagic larval stage there is considerable uncertainty about its characteristic, ancestral features. The uncertainty is mainly due to highly disparate larval types in the derived clade Neonemertea and the scarce information on the larval types of Palaeonemertea, the basal-most branching clade of Nemertea. To remedy this shortage of data the early larval development of a member of the palaeonemertean genus Carinina, Carinina ochracea, is described for the first time. Congruently to what has been described in other palaeonemertean species the larva of C. ochracea is uniformly ciliated along with an apical tuft of elongated cilia housed in an epidermal depression, the apical pit. A pair of frontal epidermal invaginations present bilaterally of the apical pit has been shown in other palaeonemertean larvae as well. Following gastrulation the blastopore is shifted to the ventral side of the body to become the mouth opening. A pair of simple unbranched nephridia that is situated on both sides of the mouth opening completes the list of structures documented for palaeonemertean larvae. Formation of the anal opening was not observed during this study. The presence of a single pigmented ventral eye was hitherto reported from the larva of Carinoma species only. The presented results complement the record of larval types found in Palaeonemertea and will serve as a reference for subsequent studies on the development of internal structures in palaeonemertean species.