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Figure 12 | Frontiers in Zoology

Figure 12

From: The nervous system of Isodiametra pulchra (Acoela) with a discussion on the neuroanatomy of the Xenacoelomorpha and its evolutionary implications

Figure 12

Phylogeny of depicted acoelomorphs amended with schemes of corresponding nervous system (red), statocysts, digestive tracts and spermatozoa (from top to bottom). (*) Hypothetical ancestor either with a bipartite brain that was not crossed by the alimentary tract and not staggered into forebrain, midbrain and hindbrain along the A-P axis but into anterior pole and axial nervous system or with a biphasic life cycle and an actively swimming and feeding larva, acoelomorphs being progenetic descendants. (1) Ring-commissure and small number of posterior neurite bundles. Adaptations to interstitial lifestyle: statocyst, frontal glands, multiciliary epidermis with special rootlet system and stepped tips of cilia, copulation. (2) Statocyst with two statoliths. Adaptation to internal fertilization: cork-screw-shaped sperm. (3) Nervous system looses basiepidermal position and tendency to develop an additional anterior ring commissure. Statocyst with one statolith, digestive system with unpolarized cells. Adaptation to internal fertilization: biflagellate sperm. (4) Posterior pharynx. (5) Female accessory organs. (6) Subterminal anterior pharynx. (7) Circular pattern of brain is abandoned and results in bilobed brains with one to three commissures. Branch-lengths estimated after[14, 17], schemes of sperm, digestive tracts and statocysts from[9, 45], pattern of nervous system from the following sources: N. westbladi[47]; D. gymnopharyngeus[48]; P. rubra[49]; H. miamia[50]; S. funilis[51]; I. pulchra [this paper]; S. roscoffensis[1012].

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