_Pentactaea Theory._--In opposition to the calycinal theory has been the _Pentactaea_ theory of R. Semon. There have always been many zoologists prepared to ascribe an ancestral character to the holothurians. The absence of an apical system of plates; the fact that radial symmetry has not affected the generative organs, as it has in all other recent classes; the well-developed muscles of the body-wall, supposed to be directly inherited from some worm-like ancestor; the presence on the inner walls of the body in the family _Synaptidae_ of ciliated funnels, which have been rashly compared to the excretory organs (nephridia) of many worms; the outgrowth from the rectum in other genera of caeca (Cuvierian organs and respiratory trees), which recall the anal glands of the Gephyrean worms; the absence of podia (tube-feet) in many genera, and even of the radial water-vessels in _Synaptidae_; the absence of that peculiar structure known in other echinoderms by the names "axial organ," "ovoid gland," &c.; the simpler form of the larva--all these features have, for good reason or bad, been regarded as primitive. Some of the more striking of these features are confined to _Synaptidae_; in that family too the absence of the radial water-vessels from the adult is correlated with continuity of the circular muscle-layer, while the gut runs almost straight from the anterior mouth to the posterior anus. Early in the life-history of _Synapta_ occurs a stage with five tentacles around the mouth, and into these pass canals from the water-ring, the radial canals to the body-wall making a subsequent, and only temporary, appearance (fig. 4). Semon called this stage the _Pentactula_, and supposed that, in its early history, the class had passed through a similar stage, which he called the _Pentactaea_, and regarded as the ancestor of all Echinoderms. It has since been proved that the five tentacles with their canals are interradial, so that one can scarcely look on the _Pentactula_ as a primitive stage, while the apparent simplicity of the _Synaptidae_, at least as compared with other holothurians, is now believed to be the result of regressive changes. The _Pentactaea_, at all events as it sprang from the brain of Semon, must pass to the limbo of mythological ancestors.