SANTAGATA, S.*; ZIMMER, R.L.: Muscle, neural, and epithelial tissue variation between larval and presumptive juvenile structures in different species of actinotroch larvae (Phoronida).

Actinotroch larvae exhibit variation among several characteristic traits at competence. These traits include the structure of the larval and juvenile tentacles and the muscular and neural components of the hood. All actinotrochs develop three median hood nerves at competence, however there are at least two distinct types of neural structures associated with these nerves among different species. One is a secondary neuropil and the other is a secondary nerve complex that contains scattered serotonergic cell bodies. Larval and presumptive juvenile tentacles show different degrees of cellular integration among species. Currently there are three known designs for the origin of the juvenile tentacles: the larval tentacles are remodeled into the juvenile tentacles, the juvenile tentacles develop from a basal primordium attached to the larval tentacles, or the juvenile tentacles are developed as a separate set underneath the larval tentacles. All species from the two latter groups form lateral cilia on their juvenile tentacle primordia, but these cilia remain immotile during larval life. The juvenile tentacular epithelium of one of these species also selectively labels for serotonin. Larval categories based on these characteristics do not directly correlate with generic groupings or reproductive types of the adults except in the case of Phoronis architecta and P. muelleri. These data also do not support the proposed synonymy of P. architecta with P. psammophila.