STRUCTURE AND FUNCTION OF THE FRONTAL GLANDS IN TURBELLARIA ACOELA AND MACROSTOMIDA (PLATYHELMINTHES) (MUCUS, GARDENING PRINCIPLE, ULTRASTRUCTURE, ATTRACTION, CILIARY GLIDING)

Present models of turbellarian evolution depict the ancestral organism with a frontal organ and therefore imply that this organ is homologous throughout the Turbellaria. Comparisons of representatives of Acoela and Macrostomida, two putatively primitive orders, show that frontal organs in these two are not similar in ultrastructure or histochemistry. The acoel Convoluta sp. had a prominent cluster of frontal mucous glands whose necks emerged together in a frontal pore at the exact apical pole of the organism, and an array of smaller glands of at least five other types which opened at the anterior end, separately from and ventral to this pore. The 'frontal organs' of seven species of Macostomida comprised an array of discretly emerging necks of at least two gland types neither of which emerged solely at the apical pole. Even though two or three types of sensory receptors were found distributed over the anterior end of all species examined, frontal gland complex did not appear to be sensory. Because of uniformity in frontal gland ultrastructure, relationships within Macrostomida based on this character alone, cannot be detected. But lack of ultrastructural, positional, and histochemical similarities between the two orders clearly indicates a low probability of homology.; If the frontal glands in these two orders are not homologous, they are at least analogous, both being involved in mucus production. A functional investigation of mucous secretions of Convoluta sp. revealed that the frontal organ is essential in establishing distribution patterns, in the nutrition, and in the locomotion of animals. Experiments testing choice of the animals between sand previously exposed to Convoluta sp. (natural sand) and acid-cleaned sand resulted in the preferential choice of Convoluta sp. for natural sand (Z = 2.45; P < 0.05). Because the animals were distributed gregariously in their natural environment (Morisita Index, I(,(delta)) = 2.34; F = 20.19; P < 0.05), I propose that mucus is involved in this observed spatial distribution pattern by acting as an attractive signal to conspecifics. I also found that these mucous secretions agglutinate sediment particles, thus promoting the stabilization of the sediment. Secreted mucus significantly enhanced the growth rate of at least one food item of the acoel, the diatom Nitzschia curvilineata (U = 25; P < 0.01). Finally, using 0.2 M dithiothreitol it was shown that viscous mucus might also be a requirement for ciliary swimming.