Royal Society. 471 



attached by an elastic ligament. The mallei are moved to and fro 

 by distinct muscles, which the author describes in detail ; and by 

 the action of these they approach and recede alternately ; the rami 

 opening and shutting simultaneously, with a movement derived 

 partly from the action of the mallei, and partly from their own 

 proper muscles. 



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 accessoiy organs. The 

 ciliated disc has an infundibuliform centre, which commonly merges 

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

 float in the water, or swimming animalcules, are whirled by the 

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

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

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

 enter the oesophagus, which opens immediately beneath them. 



From this normal condition, the author traces the manducatory 

 organs through various modifications, in the genera Euchlanis, 

 Notommata aurita. N. clavulata. Anuria, N. petromyzon, N. lacinulata , 

 Furcularia, N. gibba, Synchceta, Polyarthra, Diylena, Eosphora, Al- 

 bert ia, F. marina, Asplanchna, Mastigocerca, Monocerca, and Scaridium. 

 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 successive degenerations of the mallei, 

 and augmentations of the incus. 



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

 Pompholyx, Pterodina, (Ecistes, Limnias, Melicerta, Conochilus, Me- 

 galotrocha, 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 approaches in figure to the quadrant of a 

 sphere. 



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

 apprehension) the " stirrup-shaped " armature of the genera Rotifer, 

 Philodina, Actinurus, &c. Here, however, the organs are proved to 

 have no essential diversity from the common type ; their analogy 

 with those last described being abundantly manifest, though they arc 

 still further disguised by the obsolescence of the manubria. 



Floscularia and Stephanoceros, the most elegant, but the most 

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

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

 manubria are reduced to extreme evanescence, thougli 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 disguised, in the manducatory organs of the Roti- 

 fera, the author proceeds to the question of their homology. He 



