484 GENERAL HISTORY OF THE INFUSORIA. 



was accepted by Ehrenberg, to compreheiid the Tardigrada, the best-known 

 of which were inckided in a genus Aretiscon, so named by Schrank. His 

 opinion now is, that " perhaps they should rather be associated with the class 

 Arachnida, as a lower tj-pe, near the Acarinae," and not be numbered Tvith the 

 Crustacea, as he formerly proposed. '' Doyere's figures of Emydium indicate 

 their alliance ^dth the Acarinse, like many of which the Xenomorphidce (Tar- 

 digrada) suck the juices of other animals. Their development differs from 

 that of Rotifera ; and their skin is composed of chitin." This last distinction, 

 also insisted upon by Kaufmann, vanishes if Leydig be correct in his state- 

 ment that Rotatoria likewise have a chitinous cuticle. 



The most recent writer on Tardigrada we have met with is Kaufmann 

 (ZeitscJir. 1851, p. 220), who has presented an able memoir on those beings. 

 He indicates the following distinctive features between them and Rotatoria : 

 — The history of their development accords with that of Arthropoda, and 

 disagrees with that of Rotifera : the epidermis is composed of chitin, a sub- 

 stance only found in Arthropoda (this we have already stated is probably an 

 error) ; the pairs of indistinctly -jointed limbs and the abdominal chain of 

 ganglia no Rotifer possesses, whilst, on the other hand, the Tardigrada have 

 no trochal disk and no vibratile cilia, but possess a suctorial mouth ; lastly, 

 they are deficient of a water- vascular system, and are all hermaphrodite. 



Cohn, in a recent paper (Siebold's Zeitsclirift, 1855, p. 481), throws some 

 doubt on this presumed monoecious nature of the Tardigrada. Thus, he says, 

 Doyere, whilst maintaining their hermaphrodite character, has noticed seminal 

 corpuscles (spermatozoa) in only two individuals. On the other hand, he men- 

 tions certain examples in which the oral organs were aborted, and both sucto- 

 rial disk and maxillary head were wanting ; this happened most frequently 

 in 3Iacrobiotus Hufelandii, and more rarely in other species. Another notable 

 fact is, that in the two closely-aUied species, Macrohiotus Hufelandii and 

 Macr. Oberhauserii, the ova of one are thick-shelled and tubereulated, and 

 those of the other thin-shelled and smooth. In these circumstances Cohn 

 is disposed to find a parallel between Tardigrada and Rotatoria in what 

 relates to their sexual peculiarities, — inferring by this, that, as in the latter 

 family the sexes are separated, and ova of three sorts — male, " summer " 

 (asexual), and ''winter" — are produced, so, from the facts indicated, the 

 Tardigrada may also be bisexual (dioecious) and may deposit eggs of each 

 several kind. 



The relation of Tardigrada to Arachnida through the lowest divisions of 

 the latter, Kaufmann proceeds to demonstrate by the following particulars : — 

 They have suctorial mouths, like most Acari ; in the structure and disposition 

 of the digestive organs they agree with Arachnida ; by the absence of circu- 

 latory and respiratory organs they are aUied to the Acarina in part, and to 

 the Pycnogonidce entirely ; lil?:e many mites (Acarina), they lay few and 

 large eggs. But, again, the occurrence of a metamorphosis to some extent 

 detaches them from the Pycnogonidce and from most Acarina ; and they 

 differ fi'om all Arachnida by being hermaphrodite ; however, the circumstance 

 of the separation of the sexes, or their union in the same indi-s^idual, in no 

 class of animals can supply the basis for constituting family distinctions. 

 Even among Arthropoda a family of hermaphrodite animals occui's, viz. the 

 Cirripedia. In this respect the Cnistacea and Arachnida, by their lowest 

 members, through which they are linked to other classes of animals, accord ; 

 in the former the Cirripedia, which ally them with the Mollusca, — in the 

 latter the Tardigrada, which approximate the Arachnida to the Annelida, 

 bring the two into connexion. 



The conclusion therefore is, that the Tardigrada constitute the lowest 

 section of the Arachnida, by the side of the Pycnogonidce and the Acarina. 



