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ROTIFERA. 



ROTIFERA. 



ROTIFERA, Wheel-Animalcules, a class of animals placed by 

 Ehrenberg among the Infusoria, under the name of Rotatoria. They 

 have acquired these names on account of the apparent rotation of the 

 disc-like organs which surround their mouths and which are covered 

 by cilia. These creatures are very minute, and although some of the 

 larger forms may be detected by the naked eye, their organisation 

 can only be seen by the aid of the microscope. We are indebted to 

 Leeuwenhoek for the discovery of their existence and the first 

 account of their structure and habits. In the ' Philosophical Trans- 

 actions ' for 1702, he gave an account of the discovery of what is now 

 called Rotifer Bulgaria, one of the most common forms of these 

 animals. He afterwards described another form, Melicerta ringens. 



Subsequent observers added to the discoveries of Leeuwenhoek, so 

 that in 1824 Bory St. Vincent described 80 species. In 183S Ehrenberg 

 published his great work on the Infusoria, and there describes 189 

 species of Rotifera in 55 genera. Although classed by Ehrenberg 

 with the Polyyattrica, their organisation is much higher and more 

 complicated, and the only claim they appear to have to be classed' 

 together is the fact of their minute size. [INFUSORIA.] 



The Rotifera are very widely diffused on the surface of the earth. 

 They inhabit both fresh-waters and the ocean, and are found in the 

 cold, temperate, and tropical parts of the earth. Although capable 

 of swimming freely they are generally found near or attached to the 

 leave* of plants. They are found constantly present in ponds and 

 stream 4 in which Ceratophyltum, Callitriche, Valieneria, and other 

 fresh-water plants abound. 



A curious point in their history, which first attracted the attention 

 of Leeuweuhoek, is their power of retaining their vitality after having 

 been more or leak perfectly desiccated. This property is undoubtedly 

 possessed by the ova of the lower animals, especially those which are 

 called ' winter eggs ; ' but it does not appear to be very generally pos- 

 sessed by animals of an organisation as high as the Rotifera. Professor 

 Owen states that he has observed the revivification of one of these 

 wheel animalcules after having been kept four years in dry sand. 



The Rotifera have usually an elongated form, although some of them 

 are nearly as broad as they are long. In most instances they are 

 covered with a lorica or double envelope, the outer layer of which is 

 often of a horny consistence. Some of them build for themselves a little 

 case, or tube, in which they live, but this must not be confounded 

 with their proper envelope. When the lorica is soft the animal has 

 considerable power of elongating and contracting its body, as seen in 

 Rotifera vulgaris. Many of the species are furnished with an elongated 

 tail, which is often supplied with pincer-iike organs, to enable them to 

 remain stationary whilst feeding. Those which form tubes are 

 usually fixed. 



The rotatory organs, or wheels, are fleshy retractile lobes, covered 

 with vibratile cilia, and are capable of being contracted or expanded. 

 These organs are moved by means of a muscular system. Muscular 

 bands are observed attached to the tegumentary system, and also to 

 that part of the digestive system connected with the rotatory organs. 

 The fibres of these muscles have been observed to present the striae 

 which are characteristic of voluntary muscles. 



The digestive system consists of a mouth, jaws, frequently a dila- 

 tation which may be regarded as a stomach, and an intestinal tube 

 which has an anal orifice. The jaws generally consist of two semi- 

 circular pieces, to which are attached one or more tcetb, which act 

 upon a central plate. The number of teeth varies, and also the form 

 and character of the jaws in different species. 



The Rotifera have no true circulating or respiratory organs, although 

 Ehrenberg has described certain parts of their structure as such. In 

 most of the species minute vessels can be seen, which terminate in 

 blind sacs or caeca. In these caecal branches a vibratile body exists, 

 which keeps up a peculiar flickering movement, and it is to these 

 bodies that Ehrenberg has given the name of ' gills.' This system of 

 Teasels is regarded by Mr. Huxley and other observers as a true 

 aquiferous or water-vascular system. Connected with the respiratory 

 apparatus, according to Ehrenberg, is an organ projecting from the 

 under surface of the mouth, which he has called the 'calcar,' 'siphon,' 

 or ' respiratory tube.' Ehrenberg describes it as a tube, and supposes 

 currents to pass from it. It is connected with the nervous ganglion. 

 Dujardin and Huxley have not observed either currents or an orifice 

 in this organ. 



All observers agree that the Rotifera possess a nervous system, which 

 presents itself in the form of a pair of cephalic ganglia, from which 

 proceed nervous filaments. The extent of the development of the 

 nervous system is a subject for further inquiry. The red spots which 

 Ehrenberg calls eyes are subject to considerable variations in appear- 

 ance. Mr. Huxley says he observed them in young Lacinularue, but 

 not in adult individuals. 



The existence of sexes in a species of Notommata has been clearly 

 made out by Mr. Brightwell of Norwich. The male however is much 

 smaller and less developed than the female. All observers agree that 

 tho parts to which Ehrenberg has assigned the functions of male 

 organs are not so. Certain caudate bodies have been described by 

 K'jlhker as Spermatozoa, but their nature is doubted by other observers. 

 Mr. Huxley describes in Lacinularia certain " vacuolar thickenings," 

 which he suggests have been previously mistaken for male organs, 

 ganglia, Ac. Ovarial organs are easily made out in most of the species. 



HAT. HJ8T. DIT. VOL. IV. 



The ova are of two kinds. Mr. Huxley says in Lacinularia they con- 

 sist, first, of bodies which resemble true ova in their origin and subse- 

 quent development, and which possess only a single vitellary membrane ; 

 second, of bodies half as large again as the foregoing, which resemble 

 the ephippium of Daphnia : like it they have altogether three invest- 

 ments, and which do not resemble true ova either in their origin or 

 subsequent development ; which therefore probably do not require 

 fecundation, and are thence to be considered as a mode of asexual 

 reproduction. Professor Williamson has described very fully the 

 development of the true ova in Melicerta ringens, from which it appears 

 tho young after they are hatched do not pass through any of those larval 

 changes which are characteristic of animals both higher and lower iu 

 organisation. All the changes which take place occur in the ovum. 



The position of the Rotifera in relation to the other forms of animal 

 life has been the subject of much discussion. Dr. Grant was one of 

 the earliest writers to take them out of the Radiata, and place them 

 amongst Articulata. The relation of such forms as Stephanoceros to 

 the Ciliobrachiate Polyps is very evident. In his ' Memoir ou Lacinu- 

 laria,' Mr. Huxley gives his reasons for regarding these creatures as 

 permanent larva-forms of Echinodermata. After referring to the various 

 forms of Rotifera, and their homologous organs, he thus concludes : 



" We may say, therefore, that the Rotifera are organised upon the 

 plan of an Annelid larva, which loses its original symmetry by the 

 unequal development of various regions, and especially by that of the 

 principal ciliated circlet or trochal band ; and it is curious to remark 

 that, so far as the class of the Rotifera can be considered to be made 

 out (approximately), the dioecious forms belong to the latter of the 

 two modifications of the type which have been described, while the 

 monoecious forms belong to the former. 



" It is this circumstance which seems to me to throw so clear a light 

 upon the position of the Rotifera in the animal series. In a Report 

 in which I have endeavoured to harmonise the researches of Professor 

 Muller upon the Echinoderms, ' Annals of Natural History,' 1851, I 

 have shown that the same proposition holds good of the latter in 

 their larval state, and hence I do not hesitate to draw the conclusion 

 (which at first sounds somewhat startling) that the Rotifera are ths 

 permanent forms of Echinoderm larvae, and hold the same relation to 

 the Echinoderms that the Hydriform Polypi hold to the Medusa, or 

 that Appendicularia holds to the Ascidians. 



" The larva of a Sipunculus might be taken for one of the Rotifera ; 

 that of Ophiura is essentially similar to Stepliawoceros ; that of Asterias 

 resembles Lacinularia or Melicerta. The pre-trochal processes of the 

 Asterid larvae Brachiolaria are equivalent to those of Brachionus. 



" Again, the larvae of some Asterid forms and of Comatula are as 

 much articulated as any Rotifera. 



" It must, I think, have struck all who have studied the Echinoderms, 

 that while their higher forms, such as Echiurus and Sipunculus, tend 

 clearly towards the dioecious Annelida, the lower extremity of the 

 series seemed to lead no-whither. 



" Now, if the view I have propounded be correct, the Rotifera furnish 

 this wanting link, and connect the Echinoderms with the Nemertidte 

 and Nematoid worms. 



"At the same time it helps to justify that breaking up of the class 

 Radiata of Cuvier, which 1 have ventured to propose elsewhere, by 

 showing that the Rotifera are not ' radiate ' animals, but present a 

 modification of the Annulose type belong, in fact, to what I have 

 called the Annuloida, and form the lowest step of the Echinoierm 

 division of that sub-kingdom." 



Dr. Leydig, in a paper in the 'Zeitschrift fur Wissenschaftliche 

 Zoologie," vol. VL, on the other hand, regards the relations of Rotifera, 

 as much more with the Crustacea than with the Worms. The points 

 of resemblance to which he draws attention are : 



1. Their external figure and hard tegument, which more nearly 

 resembles the carapace of the Crustacea than the rings of the A rticulata. 



2. Their muscular structure, which resembles that of many Crustacea. 



3. Their nervous system resembles that of many Entomostraca, as 

 Daphnia. 



4. The alimentary canal resembles that of Daphnia. 



5. The resemblance in the character of their ova, the Entomoitraca 

 having two kinds of ova, as the Rotifera. 



6. The development of Rotifera and Entomostracous Crustacea 

 correspond. 



Leydig concludes a very able paper by proposing to call the Rotifera 

 Ciliated Crustaceans. 



With regard to the arrangement of the Rotifera, we give that of 

 Ehrenberg, which, although exceedingly defective, served to make 

 these creatures known amongst naturalists : 



Order 1. Rotaloria nuda. Order 2. Rotatoria loricata. 



Section I. Monotrocha. 



Ciliary Circle simple and entire, and not variable. 

 Monotrocha nuda. Monotrocha loricata. 



1st Fam. Ichthydina. 

 A. No eyes. 



a. Body smooth. 

 * Tail simple, truncated, 

 and flexible. 

 Gen. Ptygura. 



2s 



