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^. 



Fig. 5. Siphonaria. 



^^^^^^^ LI 



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. Loud., 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 obser^•e^s that 

 a blood-proper system does not exist in the Rotifera. These ani- 

 malcules are provided with a rudimentary water vascular system. 



