170 PROCEEDINGS OF SOCIETIES. 



All these organs have great solidity and density ; and, from the 

 action of certain menstrua upon them, appear to be of calcareous 

 origin. 



The writer proceeds to describe the accessory organs. The 

 ciliated disc has an inAindibuliform centre, which commonly merges 

 into a tube before it enters the mastax. The particles of food that 

 float in the water, or swimr.iiiig animalcules, are whirled by the 

 ciliary vortex into this tube, and being carried into the mastax are 

 lodged upon the rami, between tlie two unci. These conjointly 

 work upon the food, which passes on towards the tips of the rami, 

 and enter tlie oesophagus, which opens immediately beneath them. 



From tins normal condition, the autlior traces the manducatory 

 organs lhruu'j;ii various modifications in the genera Euchlanis, 

 Notommata aurita, N. clavulata, Annrcea, N. petromyzon, N. laci- 

 nulata, Furcularid, N. gibba, Synclireta, Polyarthra, Diglena, 

 ,Eosphora, Albertia, F- marina, Asplanchna, Mastigocerca, Mono- 

 cerca, and Scaridiiim. Some of these display peculiarities and 

 aberrations highly curious. Notwithstanding the anomalies and 

 variations which occur, however, the same type of structure is seen 

 in all ; and the modifications in general may be considered as suc- 

 cessive degenerations of the mallei, and augmentations of the incus. 



The form of the manducatory organs, which occurs in Triarthra, 

 Pompholyx, Pterodina, CEcistes, Limnias, Melicerta, Conochilus, 

 Megalotrocha, Lacinularia, and Tubicolaria, is next examined. 

 The organs are shown to be essentially the same as in the former 

 type, but somewhat disguised by the excessive dilatation of the 

 mallei, and by the soldering of the unci and the rami together, into 

 two masses, each of which approaclies in figure to the quadrant of a 

 sphere. 



Attention is then directed to what has been called (but by a mis- 

 apjirt'hension) tlie "stirrup-shaped'' armature of the genera Rotifer, 

 Philodiua, Actinurus, &c. Here, liowever, the organs are proved 

 to have no essential diversity from tlie conmion type; their analogy 

 with those last described being abundantly manifest, though they 

 are still furtlier disguised by the obsolescence of the manubria. 



Flsculuria aiid Stephanoceros, tiie most elegant, but the most 

 aberrant forms of R ttifera, close the series. The mastax, in these 

 genera, is wanting ; and in the former genus the incus and tlie 

 manubria are reduced to extreme evanescence, though the two-fin- 

 gered unci show in their structure relative position and action, the 

 true analogy of these organs. 



Having thus shown that there is but one model of structure, how- 

 ever modified or d.isguise i, in the manducatory organs of the Roti- 

 ferd, the author proceed-; to the question of their homology. He 

 argues on several grounds that they have no true affinity with the 

 gastiic teeth of the Crustacea, though he states his conviction that 

 the Rotifera belong to the great Arthropodous division of animals. 



It is with the Insecta that the author seeks to ally these minute 

 creatures; and, by a course of argument founded on the peculiarities 

 of structure already detailed, he maintains the following identifica- 

 tions : — that the mastax is a true moulh ; that the mallei are man- 



