Comparative Ultrastructure and the Evolution of Nemertines

Abstract

Nemertines are traditionally viewed as acoelomates with blood vascular systems and a specialized cavity housing an eversible proboscis, or, as coelomates with modified coelomic cavities forming the rhynchocoel, lateral vessels and gonadal sacs. Comparative ultrastructural data such as cell polarity, the occurrence of podocytes and mesodermal cells with rudimentary cilia for proboscis, rhynchocoel, lateral and dorsal vessels, and gonadal linings are consistent with their interpretation as modified coelomic cavities as found, analogously, in the higher leeches. These and additional data regarding musculature, connective tissue, glia-like cells and gametes for 13 species affirm that nemertines are intermediate between full-blown coelomate and acoelomate grades of organization. A comparison of ultrastructural details, however, indicates that their acoelomate attributes evolved secondarily from a more typical coelomate condition.