On the Mechanism of Aquatic Respiration. 333 than broadj in straight cross lines, with a broad rounded lobe, rather more sinuous on the inner than on the outer side of its front edge. SlPHONARIAD^. Sipkonaria. The teeth of Siphonaria are on a broad, rather long, dark brown lingual band, are numerous, equal, in a slightly arched cross line ; the central tooth is narrow, elongated, with a small rhombic apex ; the lateral teeth are larger, diverging, and gra-dually diminish in size towards the outer side of the series, and furnished with a rather oblique curved tip (see fig. 5). Fig. 6. Cyclophorus. The teeth of Cyclophorus Inca (fig. 6) are similar to those of Natica and the other marine genera belonging to the group of Ptenoglossa. XXXIII. — On the Mechanism of Aquatic Respiration and on the Structure of the Organs of Breathing in Invertebrate Animals. By Thomas Williams, M.D. Lond., Licentiate of the Royal College of Physicians, formerly Demonstrator on Structural Anatomy at Guy's Hospital, and now of Swansea. [With a Plate.] [Continued from p. 261.] Rotifera. — It is the undivided belief of all recent observers that a blood-proper system does not exist in the Rotifera. These ani-malcules are provided with a rudimentary water vascular system^
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