Gastrulation and early actinotroch of Phoronopsis harmeri

This is a side view of a blastula of the phoronid, a.k.a. horseshoe worm, Phoronopsis harmeri. It is about 22 hours after egg release (developing at ambient sea temperature).  Eggs in this species are fertilized inside the body cavity, but initiate their development after spawning (or being dissected, as is the case here).  Note the spacious cavity called blastocoel inside the blastula. The surface of the blastula is ciliated, each cell bearing a single cilium. Longer cilia at the animal pole (up) indicate position of the future apical sensory organ. 

This is a gastrula stage. It is 24 hours old.   The circle of cells inside the blastocoel is the developing gut of the embryo, otherwise known as the archenteron.  The archenteron opens to the outside via the blastopore which will later develop into the mouth. Cells clustered at one end of the archenteron are the mesoderm cells which will form the muscles and coelomic sacs in the larva. 

This is a young actinotroch larva that is ready to feed on microscopic algae. It has a complete gut with mouth and anus. The mouth opens under the anterior hood and leads into a spacious vestibule, which leads into the gut. Note a thickened region of epidermis directly overlaying the vestibule - this is the apical sense organ. Sandwiched between the apical sense organ and the vestibule is a thin-walled sac - this is the anterior coelomic cavity, the protocoel. The gut proper has two distinct compartments - the stomach, which occupies most of the actinotroch’s body, and the short hindgut that opens via an anus at the posterior end. The actinotroch will later develop a crown of tentacles posterior to the mouth which will assist in capturing food.