This photo shows a six-day-old actinotroch larva of the phoronid worm Phoronopsis harmeri. Our class collected the adults from a mudflat just north of the Charleston bridge in Charleston, OR. Our instructor Dr. Svetlana Maslakova started a culture the week before our phoronid day, so that we had the opportunity to observe advanced developmental stages. Phoronopsis harmeri have separate sexes. The fertilization is internal, so that when one dissects a female, the eggs that fall out of the body coelom are already fertilized.
On this picture one can see the hood (at the anterior end of the larva, which is oriented up). One can also see the developing tentacular ridge (looks like a bump on the ventral side, which is to the right). Otherwise one can clearly see the gut, which consists of several regions. The mouth, which opens under the hood leads into the vestibulum, which is the large cavity under the hood. The vestibulum leads into the esophagus (or foregut), which looks like a wide tube with thick but colorless walls. The esophagus leads into the stomach. The stomach occupies the majority of the space inside the trunk of the larva and is colored orange-brown. The pigment is derived from the algal food. The stomach leads into the midgut (a short colorless region at the posterior end of the stomach), which in turn leads to the proctodaeum (or hindgut), a short pink-colored tube which opens via the anus at the posterior end of the body. See a more advanced larva of this species collected in plankton.