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each uncus rest upon the corresponding ramus, to which they are 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 accessory 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 œsophagus, 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, Anuræa, N.petromyzon, N. lacinulata, Furcularia, N. gibba, Synchæta, Polyarthra, Diylena, Eosphora, Albertia, 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, Œcistes, 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 approaches in figure to the quadrant of a sphere.
Attention is then directed to what has been called (but by a misapprehension) the "stirrup-shaped" armature of the genera Rotifer,2 G 2
