VI 



TARDIGRADA 



545 



and the oesophageal bulb an oesophagus is intercalated. Two blind 

 tubes enter the rectum, no doubt corresponding with the Malpighian 

 vessels of the Tracheata. 



The sexes are separate ; the germ glands unpaired, and sac- 

 shaped. In both sexes they enter the hind -gut, which thus becomes 

 a cloaca. Special circulatory and respiratory organs are wanting. 

 The nervous system consists of a brain, an infra -oesophageal gan- 

 glion which is connected with the brain by two oesophageal commis- 

 sures, and 4 other ventral ganglia, which are connected by longitudi- 

 nal commissures placed far apart from one another. There are two 

 eye spots in the head, lying on two 

 small ganglia connected by nerves with 

 the brain. The musculature is richly 

 developed. Various dorsal, ventral, 

 and lateral longitudinal muscles run 

 under the integument. Special muscles 

 serve for moving the legs. All the 

 muscles are smooth. 



The systematic position of the Tardigrada 

 is uncertain. The three related facts that 

 they possess accessory organs of the hind-gut 

 comparable with the Malpighian vessels, tube- 

 like oral glands, and truncated feet provided 

 with claws, make it not improbable that 

 they belong -to the Arthropoda, and especially 

 to the Tracheata. The want of oral limbs, 

 the structure of the nervous system, and the 

 manner of emergence of the sexual organs, 

 stand in the way of a comparison of the Tar- 

 digrada, with the Acarina. Although we 

 may agree with the view that they are some- 

 how related to the Tracheata, or to the ances- 

 tors of the Tracheata, we cannot in any case assume that they, in any way, resemble 

 the primitive arrangements. The want of a blood-vascular system, the unpaired 

 germ glands, the reduced and abbreviated condition of the whole body, the absence 

 of nephridia and of coxal glands (?), rather make the Tardigrada appear as a one- 

 sidedly developed lateral branch. 



Most of the Tardigrada live among moss and lichens, a very few in fresh or salt 

 water. They can stand desiccation, and remain apparently dead for a long time, 

 reviving again when wetted. Echiniscus, Macrobiotus, Milnesium, Doyeria. 



Fig. 383. Posterior portion of the 

 body of Macrobiotus Hufelandii $ , from 

 the side, h, Testis ; ad, accessory gland of 

 the male sexual apparatus ; d, cloaca ; an, 

 anus ; vm, excretory tube (Malpighian 

 vessel) ; md, mid-gut (after Plate). 



Literature. 



Ludwig H. Plate. Beitrage zur NaturgescMchte der Tardigraden. Zool. Jahrbucher 

 von Spengel, Abth.fur Anatomic und Ontogenie. 3 Bd. 1888. In this treatis 

 there is a bibliography up to 1888. 



VOL. I 



2 N 



