The Embryology of the Polychaete Scoloplos artniger
Authors: Anderson D. T. 
Published: April 2021

The eggs of Scoloplos armiger, laid in gelatinous cocoons, are yolky. Spiral cleavage and gastrulation produce a yolky trochophore, which then develops 12 (embryonic) trunk segments in strict succession. Ciliated bands occur, though development isnon- pelagic. Segment-formation ceases after 8 days and the embryo hatches and metamorphoses into a feeding ρre-adult, cilia being lost and yolk resorbed. Further (post- embryonic) segments are now added to the trunk. Anterior to the first trunk segment lie the prostomium and mouth region, derived from the main body of the trochophore. Posterior to the trunk segments is a prepygidial growth zone, followed by a terminal pygidium.

The 12 embryonic trunk segments each contain a pair of hollow mesodermal somites descended from 4d, with ectoderm derived from 2d. The endoderm, of 3A-3C and 4D origin, meets the stomodaeum anteriorly and the pygidial ectoderm posteriorly.

The prostomium arises from part of the apical plate, derived from ia-1d, and includes the cerebral ganglion. The ectoderm of the mouth region, partly apical plate, partly 2d in origin, includes transitorily the prototroch cells. Stomodaeum and proboscis apparatus develop in the mouth region from second and third quartette cells. The stomodaeum forms the pharynx and part of the oesophagus.

An anterior median and three pairs of lateral coelomic cavities arise in the 4d mesodermal bands in the prototrochal region (prostomium and mouth region), soon becoming confluent with each other and with those of the first two trunk segments as the adult head coelom.

The ectoderm of the embryonic segments forms epithelium, chaetal sacs, and ventral nerve ganglia. The endoderm forms oesophagus, stomach, intestine, and rectum. The somites give rise to peritoneum, trunk musculature, and blood-vessels.

The post-embryonic segments have a similar origin and development to the embryonic. The mouth region and first trunk segment fuse to form the peristomium. The pygidium, a 2d ectodermal structure, undergoes little change.

A trochophore stage occurs in development in all polychaetes, variously modified according to size and yolkiness of the egg. The simple planktotrophic condition appears to be primitive. The trochophore body comprises a prototrochal region, growth zone (giving trunk segments) and pygidium.

In polychaete trunk segments, 2d ectoderm forms epithelium, chaetal sacs, and ventral ganglia. Each segment contains a pair of hollow mesodermal somites, usually of 4d origin, segmenting before the ectoderm. In Nereidae and Serpulidae are found specializations associated with heteronomy of the segments, a secondary phenomenon in polychaetes. The walls of the somites form peritoneum, trunk musculature, and blood-vessels. The haemocoel corresponds to the blastocoel.

The polychaete gut arises from stomodaeal and endodermal rudiments. In yolky forms differentiation is delayed. Proboscis development differs in different families.

The polychaete pygidium is an ectodermal post-segmental structure bearing the anus.

The polychaete head is variable and specialized. Prostomium and mouth region, derived from the prototrochal region of the trochophore, are probably presegmental. The mouth region usually combines with the first trunk segment to form the peristomium. No direct comparison can be made of the polychaete with the onycho- phoran-arthropod head.