Biology/Natural History: 18s mtDNA sequences show that the Padilla Bay and Willapa Bay populations are virtually genetically identical to one another, implying that they were transplanted to Padilla Bay from Willapa Bay relatively recently (they were reported in Willapa Bay in 1910 (after oysters were introduced there from the east coast) and described there by Woodwick, 1951). The WA specimens were identical to the Maine specimens in their 18s mtDNA but differed from the Maine specimens by a 3 base pair difference in the 16s mtDNA sequence. The mouth is on the ventral side at the junction of the proboscis and collar, usually hidden in the groove which separates the proboscis/collar. The gills are in the anterior part of the trunk, which has gill pores in this section. The pores may be elevated, or may be sunken into a longitudinal depression. The trunk also has a middorsal and a midventral ridge. Sexes are separate but difficult to distinguish. Eggs are large (up to 1 mm) and yolky in Family Harrimaniidae, which also has direct development--no swimming (tornaria) larva. S. pusillus near San Diego was found to stick the eggs to the side of the burrow (or these may be a remnant left behind after spawning).
An isolated Enteropneust proboscis and collar lives and moves for some time but does not apear to regenerate. Pieces of the trunk can regenerate into an entire individual.This species seems to avoid bright light, an observation also noted in Hyman, 1959.Enteropneusts are so similar to one another that no Orders have been set up within the Class (Human, 1959)Enteropneusts burrow by using the proboscis. This is elongated and thrust forward, then contracted longitudinally to form a bulge. This bulge is moved posteriorly to the base of the proboscis, pulling the animal forward. The collar and trunk move forward passively. Cilia on the surface of the proboscis and collar also move sand posteriorly.