Entoproct life-cycles and the entoproct/ectoproct relationship

Abstract

The larvae of several entoproct species and their metamorphosis or budding are described and the literature on entoproct development reviewed. Most species probably have spiral cleavage; some larvae, such as those of Loxosomella elegans and Loxosoma pectinaricola, are rather schematic trochophores, whereas most other larvae are highly characteristic, having a frontal organ and a large ciliated foot. The larvae of species such as Loxosomella harmeri undergo a metamorphosis; they attach by means of the contracted frontal organ and the alimentary canal rotates 90° in the median plane with the mouth first. Other loxosomatids, such as Loxosomella leptoclini and L. vivipara, have no metamorphosis; buds develop from the episphere of their larvae and the larvae die after the liberation of the buds. Loxosomella polita, in which the metamorphosed larvae never grow to a normal size or develop the differentiated “foot” characteristic of the adult specimens which have originated from buds, represents an intermediate type. The larvae of the pedicellinids attach with the area above the contracted prototroch, becoming glued to the substratum by the secretion of the three pairs of foot glands. Their alimentary canals undergo a 180° rotation. The literature on life-cycles of the three ectoproct groups Eurystomata, Cyclostomata and Phylactolaemata is reviewed. Although several problems are in need of further investigation, it seems clear that the three groups have comparable life-cycles and that the eurystomes represent the least specialized type. The origin of the Entoprocta is discussed, and it is concluded that they are related to other groups having trochophore larvae. The entoprocts having larvae which settle with the frontal organ are considered primitive and may be derived from a trochophore-like ancestor. The other types of development are regarded as caenogenetic specializations. The theories about the origin of the Ectoprocta are discussed. The derivation from a phoronid-like ancestor is rejected on the basis of a comparison of metamorphoses and budding patterns in the two groups. It is suggested that the ectoprocts have developed from entoproct-like ancestors which have had a metamorphosis similar to that of the pedicellinids and internal budding like the adults of Loxosomella bocki and the larvae of Loxosomella vivipara and Loxosoma jaegersteni. Detailed similarities in the metamorphosis of the two types support this theory, as do the similarities in the budding and the formation of hibernaculae. The body cavity of the Ectoprocta is a “fluid skeleton” which probably has developed in connection with the evolution of a retractable lophophore. Nothing indicates that a “coelom” has been present in the entoprocts. The most important differences between the ento- and the ectoprocts seem to lie in the cleavage patterns and the feeding mechanisms, but these differences do not seem to be sufficient for separating the two groups entirely. The final conclusion is that the Entoprocta and the Ectoprocta together must represent a separate phylum, the Bryozoa.