Ultrastructure of a swimming-type larva of Barentsia gracilis (Entoprocta, Coloniales)

The study of the microscopic anatomy of the larvae of Entoprocta is interesting in the context of determining the phylogenetic position of this group. It is assumed that the comparison of the organization of larval stages of entoprocts with different life cycle stages of other taxa (bryozoans, cycliophorans, mollusks) may help to find common features in their organization, indicating possible close relationships of these groups. There are few data on the microscopic anatomy of the larvae of entoprocts. The fine structure of larvae of only two solitary species, Loxosomella murmanica and Loxosoma pectinaricola, has been described in detail. Here we described the ultrastructure of the swimming larva of colonial species Barentsia gracilis. Our data clarify some details of the structure of entoproct larvae. The cuticle of larval stages is similar to that of adults. The prototroch is represented by two rows of ciliated cells. The apical and frontal ganglia are devoid of a central neuropil, which distinguishes them from the ganglion of adults. Ganglia include not only nerve cells but also muscle cells. The digestive tract of larvae and adults is similar not only at the histological level but also at the ultrastructural level. Three pairs of pedal glands include cells with electron-dense granules and cells with vacuoles with granular contents. These glands probably secrete different components of mucus. Each of the protonephridia consists of two terminal cells that pass into their own canals, which then fuse together and open by the nephropore on the hyposphere.