OF THE TARDIGHADA. 483 



The nervous system is well developed. It consists of a chain of ganglia, 

 with intercommunicating (anastomosing) nerve-fibres, besides a central or 

 cerebral ganglion. 



The eyes are variable and fugacious. The sense of touch may be presumed 

 to reside specially about the suctorial mouth and its contiguous palpi. All 

 the Tai'digrada are hermaphrodite. The ovary is of large size ; but the ova, 

 according to KoUiker and Frey, do not in the course of development exhibit 

 a germinal disk : in this they differ from Arthi-opoda. Few eggs are pro- 

 duced at a time, and are of large size. They are, curiously enough, foimd in 

 the exuviae or moultings of the animals ; for from time to time the outer skin 

 is cast off. M. Doyere convinced himself of the exi.stence of a testis and 

 spermatozoa. Dujardin says the embryo emerges from the ovum perfect in 

 form ; but Kaufmann, on the contrary, affirms that they undergo some de- 

 gree of metamorphosis ere they attain the adult structure. 



The Tardigrada have received their name from their slow movements. They 

 are parasitic animals, and live by sucking the juices from other beings. They 

 are common upon water-plants and vegetable debris in ponds ; yet immersion 

 in water is not necessarj^, since they are found, like Rotifers, in the dust and 

 rubbish on the roofs of houses (a locality in which they were first encountered 

 by Spallanzani), and especially amid the small lichens, mosses, &c., which 

 spring up in such situations. The Bryum is a favourite moss for these crea- 

 tm'cs. On shaking portions of this or of other mosses or aquatic plants in a basin 

 of water, the Tardigrada will fall to the bottom, and may be easily collected. 



In most vital phenomena they very closely accord with Eotatoria ; thus, 

 like these, they can be revived after being put into hot water at 113° to 

 118°, but are destroyed by immersion in boiling water. They may be gra- 

 dually heated to 216°, 252°, and even 261°. It is also by their capability 

 of resuscitation after being diied that they are able to sustain their vitality 

 in such localities as the roofs of houses, where at one time they are subjected 

 to great heat and excessive drought, and at another are immersed in water. 



0. MiiUer (in 1785) seems, from the name {Acariis Ursellus) which he 

 imposed on the species he then knew of, to have rightly conceived their 

 natural affinity. Ehrenberg and Schultze (1834) placed them among the 

 Leniece. Dujardin (in 1841) advocated their alliance with the Rotatoria, and 

 constituted them one of the divisions of that class, under the name of " Sy- 

 stolides Marcheurs,^^ or creeping Rotatoria ; for he considered them to form a 

 link between the Rotatoria and the Helminthidae on one side, and the Anne- 

 hda and Arachnida on the other. M. Doyere at first coincided in this opinion ; 

 but his subsequent researches led him to give it up and to constitute the Tar- 

 digrades a distinct group. Dujardin himself has, moreover, modified his first 

 opinion, as appears by his memoir in the Annates des Sc. Nat. for 1851 ; 

 for he there remarks that the Tardigrada are equally allied to the Rotifera 

 and to the Nematoid Helminthidae, and that it is uncertain whether they 

 ought to be referred to Articulata or Vermes. Our countryman Mr. White 

 (in a paper read before the Linnean Society in 1851) stated his belief " that 

 the so-called Acarus follicidcriim, and probably also Tardigrada, are parasitic 

 Rotatoria, with legs or leg-hke appendages adapted to their peculiar habits, 

 and that their retractile, antenna-like, subtelescopic appendages may have 

 eyes passing through them, as in snails, and may also be the equivalents of 

 the rotcB (rotary lobes), but, from the limited, or rather the absolutely re- 

 stricted, power of motion of these animals, have neither the ciliary processes 

 nor the movements and economical uses of the appendages so characteristic 

 of most of the Rotatoria." 



Perty tells us that in 1848 he constructed a family XenomorpMdWf which 



2i2 



