RADIATA. 707 



indefinite period, without the loss of their vitality; being revived again by moisture, and returning to 

 their original activity. 



The reproduction of the Kotifera is not accomplished by gemmation, nor by the subdivision of the 

 body, but only by eggs. Of these, not many are produced at once ; they attain a large size in propor- 

 tion to the body of the parent (Fig. 21, it) ; and the development of the embryo often proceeds so far 

 before the egg is extruded, that its ciliary movements are visible. The egg-capsule frequently bursts 

 when the egg is passing forth, so that the young may be said to be bom alive ; being capable of active 

 locomotion, and of obtaining its own food, as soon as it quits the body of the parent. Notwithstanding 

 that the number of eggs undergoing development at one time seldom exceeds three or four, the re- 

 productive powers in this tribe are very extraordinary, in consequence of the rapidity with which the 

 young arrive at maturity, and become the parents of a new generation. From the observations of 

 Professor Ehrenberg on the rate of propagation of the Hydatina senta, one of the largest of the 

 Rotifera, he calculates that, from a single individual, more than a million might be generated in ten 

 days, and nearly seventeen millions in twenty days. This rapidity of multiplication, taken in connec- 

 tion with the power which these animals possess of being revivified after entire desiccation, is quite 

 sufficient to account for their sudden appearance in various situations in which they were not previously 

 known to exist, and for their extraordinary abundance whenever the conditions in regard to food, 

 temperature, &c., are favourable. 



The class is subdivided by Professor Ehrenberg in the first place, according to the arrangement of the ciliary 

 apparatus, into two sections, each of which contains two orders ; and every one of these four orders is subdi- 

 vided into two families, according as the external skin is soft or naked, or forms a dense lovica or sheath. 

 Section A. Monotrocha. A single, continuous, ciliated wheel. 

 Order I. — HotOTROcnA. Margins of the wheels entire. 

 Family 1. Tethydhui. Skin naked. 

 Family 2. (Ecisthia. Skin loricated. 

 Order II. — Schizotrocha. Margins of the wheels crenated. 

 Family 3. Megalotrochcca. Skin naked. 

 Family 4. FloKularia. Skin loricated. 

 Section B. Sorotrocha. A compound, or divided, ciliated wheel. 

 Order III. — Polttrocha. Many-parted wheels. 

 Family 5. Uydatiruva. Skin naked. 

 Family 6. Euchlanidota. Skin loricated. 

 Order IV. — Ztgotrocha. Two-parted wheels. 

 Family 7. PhUodinma. Skin naked. 

 Family 8. Brachionma. Skin loricated. 



HOMOGENEA. 



This group is now more commonly known under the name Polygastrica, conferred upon it by Pro- 

 fessor Ehrenberg, from the idea that the animals composing it are distinguished by the possession of 

 numerous digestive sacs or stomachs. In this view, however, many very competent observers are far 

 from coinciding ; and the previous designation may, therefore, be properly retained, until the organi- 

 zation of these beings shall have been more fully ascertained, and their true relations shall have 

 become more completely apparent. At present there is a strong tendency to transfer a large propor- 

 tion of them to the Vegetable kingdom ; it being now certainly known that an active movement, oc- 

 casioned by the vibration of cilia, is visible in many of the humbler Plants ; so that the power of thus 

 making its way through water, even with considerable activity, is by no means that certain and 

 distinctive attribute of an Animal, wliich it was at one time supposed to be. Moreover, it is certain 

 that many of them are allied to Plants in their chemical constitution, and also in their power of 

 growth and increase under the influence of light, at the expense of water and carbonic acid merely. 

 It has lately been ascertained, too, that the Desmidea; and DiatomacecE reproduce themselves by 

 conjugation, after the manner of the Zygnemata, which are undoubted plants ; so that, although these 

 are described and represented by Professor Ehrenberg as Infusory Animalcules, they are now trans- 

 ferred by most Naturalists to the Vegetable kingdom. It seems not improbable that a large proportion 



