Unique to the phylum Nemertea, the pilidium is an unmistakable planktonic larva found in one group of nemerteans, the Pilidiophora.
The pilidium is a unique larval form; its morphology is unmistakable and found in no other marine invertebrate.
On the development of the pilidium
of certain nemerteans.
It is as much diverged from the spiralian stereotype as the nemertean pilidium
, with which Owenia's development indeed shares some traits, including enlarged first quartet micromeres, gastrulation by invagination into a large blas-tocoel, and the formation of a convoluted primary ciliated band from numerous proliferative cells.
The other extreme, with highly specialized larval types with larval organs shed during a cataclysmic metamorphosis, is well known--for example, from the nemertine pilidium
larva and the larva of the annelid Polygordius.
The other two major groups of nemerteans are the stylet-bearing Hoplonemertea and the recently defined clade Pilidiophora, which is characterized by the pilidium larva and includes the Heteronemertea as well as the genus Hubrechtella.
Nemertean development is traditionally categorized into "indirect," meaning development via a long-lived plank-totrophie pilidium larva, as in most pilidiophoran species for which development is known (e.
Distribution of catecholamine-containing, serotonin-like and neuropeptide FMRFamide-like immunoreactive cells and processes in the nervous system of the pilidium
Catecholamine-containing, serotonin-like and neuropeptide FMRFamide-like immunoreactive neurons and processes in the nervous system of the pilidium
Catecholamine-containing, serotonin-like, and neuropeptide FMRFamide-like immunoreactive cells and processes of the nervous system of the pilidium
The distribution of the fluorescent lineage tracer was determined by examining the resulting pilidium
larvae with fluorescence and differential interference contrast microscopy.
Because the boundary between labeled and unlabeled ectodermal domains corresponded to the first cleavage plan, the fluorescent label made it possible to analyze the relationship between the first cleavage plane and the dorsoventral axis in the resulting pilidium