. ' -'ctr: 



March 7, 1889] 



NATURE 



441 



strange places in which they are found. For these fresh- 

 water creatures, the common inhabitants of lakes and 

 ponds, are to be found in brackish ditches, sea-pools, the 

 mud of ponds, the dust of gutters, ia tufts of moss, on the 

 blades of' wet grass, in the rolled-up leaves and in the 

 cups of liver-worts, in the cells of Volvox, the stems and 

 sporangia of Vatichena, in vegetable infusions ; on the 

 backs of Entoinostmca, on their abdominal plates, on 

 their branchial feet ; on fresh-water fleas, wood-lice, 

 shrimps, and worms ; in the viscera of slugs, earth-worms, 

 and Naiades ; and in the body-cavities of Synaptce. 



But the great variability of every part of the external 

 and internal structure of the Rotifera points to their fitness 

 for playing the parts of cosmopolites. See how in Flos- 

 cularia and Stephanoceros the head and its appendages 

 are so developed that they dwarf all the rest ; how in 

 Apsilus the trunk predominates ; while in Actinurus 

 both head and trunk become appendages of a huge foot. 

 The corona diminishes continually from the large complex 

 organs of Melicerta, Hydatina, and Brachionus, down 

 to the furred face of Adineta and the tuft of Seison, and 

 vanishes altogether in Acycliis. The antennae can be 

 traced from long infolding or telescopic tubes, furnished 

 with setiferous pistons, special muscles, and nerves, 

 through a succession of shorter and simpler structures, till 

 they become mere pimples or even setiferous pits in the 

 body surface. The skin is hardened into a perfect lorica 

 in Brachtonus, is partially hardened in Dapidia, is merely 

 tough in Mastigocerca, and is soft and quite unarmed in 

 Notommata. The appendages of the body in Pedalion 

 rise almost to the dignity of crustaceous limbs, for they 

 have joints, and are worked by opposing pairs of muscles, 

 passing across their cavities from point to point. In 

 Asplanchna these appendages become stumpy projections, 

 and the muscles, though still passing freely across the 

 body-cavity, are reduced to threads. In 7>/ar//zrrt; the ap- 

 pendages become chitinous spines ; and at last, when we 

 r^ach Adineta, Taphrocampa, and Albertia, we find that 

 We have passed from a Rotiferon closely resembling a 

 NaupUus-larva to one that is a simple worm. 



The internal structure is just as plastic. The 

 characteristic trophi exhibit a series of striking changes 

 as we pass from one genus to another. In one direction 

 the change is due to the degradation of the mallei, in the 

 other to that of the incus ; and in both this degradation is 

 pushed so far, that the changing parts may be said almost 

 to disappear. For in Brachtonus and Euchlanis the 

 mallei are well developed ; in Furcularia, mere needle- 

 shaped curved rods : in Asplanchna, so evanescent that 

 it is hardly possible to finc^ithem in an animal killed by 

 pressure. ^.V 



By another set of changes, the rami are in their turn 

 reduced almost to evanescence ; becoming feeble loops 

 in Stephanoceros, and in Floscularia two membranes 

 attached to the unci. 



Changes, great in degree, if not in variety, occur also 

 in the excreto-respiratory system. For the contractile 

 vesicle, which fills quite half the body-cavity in some 

 Asplanchnce, dwindles down in various species till it 

 seems to vanish vn. Pterodina and Pedalion; while in one 

 abnormal form, Trochosphcera, the connection between 

 the lateral canals and the contractile vesicle is snapped, 

 and the latter becomes an appendage of the cloaca only. 



The nervous system, wherever it has been made out, is 

 indeed always on the same plan ; but its central organ, 

 the nervous ganglion, is, in Copeus and Euchlanis, a 

 great cylindrical sac stretching from the head below the 

 mastax ; while in Floscularia it shrinjcs into a small star- 

 shaped body between the eyes and the organ of taste. 



The alimentary and reproductive systems are those 

 which vary the least ; but even here the difference, in 

 proportionate size, is very great between the stomachs of 

 Sacculus and Synchceta, and also between the ovaries of 

 Asplanchnopus tnyrmeleo and Asplanchna triodonta. 



But not only do most of the external parts and internal 

 organs vary in turn almost to vanishing, but these varia- 

 tions are not in any way simultaneous. The result is, 

 that we find an organ, of a form characteristic of one 

 family or genus, occurring in a species that belongs to 

 another. Thus, for instance, the trophi of the Mclicer- 

 tadcc appear in Pompholyx, one of the TriarthradcB. Nay, 

 more ; it is easy to point out Rotifera that bear some 

 striking characteristics of two or three other genera, or 

 even of two or three other families. Microdon clavus, 

 for example, has the central mouth and double ciliary 

 wreaths of one of the Rhizota,\h& eye of a Notommata, 

 the trophi of a Diglena, and the foot of a Monostyla. 

 Again, Pterodina.pnti^a has the corona oi Philodina, the 

 lorica and transversely wrinkled retractile foot of Brachi- 

 ^««j, the foot-ending of a young Rhizotan, and the mas- 

 tax of the MelieertadcE. Then there is Mr. Thorpe's 

 new Australian Floscule, which swims freely like one of 

 the Ploima, has the buccal cup and wreath of Floscularia, 

 the dorsal eye of Notommata, and the body and forked 

 foot of Proales. 



To sum up, we may say that in the female Rotifera,. 

 the corona, head, foot, toes, appendages of the trunk, 

 antennae, eyes, and contractile vesicle vary down to 

 almost absolute extinction ; while, if we include the male 

 in our survey, we must add that even the whole of the ali- 

 mentary tract may disappear also. Moreover, the charac- 

 teristics of the various groups interlace in so many ways 

 that no organ — nor, indeed, any combination of two or 

 three organs — can be relied upon to determine with 

 certainty an animal's true position. 



Two conclusions are, in consequence, irresistibly forced 

 on us : the first, that the Rotifera, from Pedalion to Al- 

 bertia, are related by descent ; the second, that their 

 curious habitats, wide dispersion and great variations in 

 their structure are due to causes that have been at work 

 for a very long period of time. 



One other fact has also been made clear in this review 

 — namely, that the British Rotifera give a very fair idea 

 of the whole class. No doubt there are many foreign 

 species, and some of these are very remarkable, and of 

 great interest ; but the greater number fall readily enough 

 into the divisions that contain our own species. 



And indeed it is a fortunate thing that we can here, at 

 our own doors, study so many typical forms from life. 

 For what books or drawings can give us the delight which 

 we derive from observing the animals themselves ? 



To gaze into that wonderful world which lies in a drop 

 of water, crossed by some atoms of green weed ; to see 

 transparent living mechanism at work, and to gain some 

 idea of its modes of action ; to watch a tiny speck that 

 can sail through the prick of a needle's point ; to see its 

 crystal armour flashing with ever-varying tint, its head 

 glorious with the halo of its quivering cilia ; to see it 

 gliding through the emerald stems, hunting for its food, 

 snatching at its prey, fleeing from its enemy, chasing its 

 mate (the fiercest of our passions blazing in an invisible 

 speck) ; to see it whirling in a mad dance to the sound of 

 its own music, the music of its happiness, the exquisite 

 happiness of living, — can anyone, who has once enjoyed 

 this sight, ever turn from it to mere books and drawings, 

 without the sense that he has left all Fairyland behind 

 him 1 



THE DARKNESS OF LONDON AIR. 



A GREAT deal has been written at various times upon 

 the subject of London fogs. 

 The constitution of these London fogs has been carefully 

 gone into by several well-known men of science, from 

 time to time ; and the results obtained are of very great 

 interest, as they prove, amongst other things, that during 

 the winter London air has an unusually large amount of 



