ИСТИНА

According to recent molecular (but not embryological) data, phoronids are true trochozoans with reduced spiral cleavage of the egg and reduced teloblasts. Using the light microscopy and histology methods we studied two phoronid species ([Phoronis ijimai] and [Phoronopsis harmeri]) to determine whether their embryonic development had trochozoan or deuterostomian characteristics. In both species, egg cleavage can generally be considered to be radial with some variations that reflect features of development. Thus, the fourth and fifth cleavage furrows in [Phoronis ijimai] are meridional; this allows for the early formation of the brick-shaped embryo, a shape that facilitates the formation of embryonic masses. In [Phoronopsis harmeri], the furrows of the forth cleavage are oblique, which allows for blastula formation by the 16-cell stage. The presence of this cavity is very important for pelagic organisms. In both species, the coelomic mesoderm originates from an anterior and a posterior precursor. The anterior precursor forms when cells migrate from the anterior wall of the archenteron at the early gastrula stage (40 h after spawning). These cells give rise to the preoral coelom and muscle cells of the preoral lobe, the esophagus, and the hyposphere. In 14-day-old larvae, these cells form the tentacular coelom. The posterior precursor appears from the dorsal pouch of the intestine in 52-h-old larvae. This precursor looks like a swelling of the dorsal wall of the proctadaeum. This pouch then separates from the intestine and transforms into a small coelomic sack (trunk coelom), which is visible on the both sides of the proctadaeum in 14-day-old larvae and which increases in volume with age. We contend that phoronids have not lost spiral cleavage and teloblasts, and have retained the plesiomorphic features of deuterostomian development such as radial cleavage of the egg and enterocoely.