482 



TARDIGRADA 



The nervous system consists of a brain or supra-oesophageal 

 ganglion, whose structure was first elucidated by Plate, and a 

 ventral chain of four ganglia. Anteriorly the brain is rounded, 



and gives off a nerve to the skin ; 

 posteriorly each half divides into 

 two lobes, an inner and an outer. 

 The latter bears the eye-spot when 

 this is present. Just below this 

 eye a slender nerve passes straight 

 to the first ventral ganglion. The 

 brain is continued round the oral 

 cavity as a thick nerve-ring, the 

 ventral part of which forms the 

 sub-oesophageal ganglion, united 

 audi, C. Sell., X about 350. (From w two longitudinal commissures 



Plate. ) Seen from the side, cqj, Lobe , . , r- j ^ t 



to the first ventral ganglion. 

 Thus the brain has two channels 

 of communication between it and 

 the ventral nerve -cord on each 



Fig. 253.— Braiu of Macrohiotus hufd- 



of brain bearing the eye ; ce, sujira 

 oesopliageal ganglion ; d, tooth ; Ga, 

 first ventral ganglion ; ga , sub-oeso- 

 phageal ganglion ; /.■, thiclieningof the 

 epidermis round tlie mouth ; oc, eye- 

 spot ; oe, oesophagus ; op, nerve run- 

 ning from the ocular lobe of the brain side, ouc by means of the slender 



to the first ventral ganglion ; ph. i „ , • i -i 



pharynx nervc above mentioned, and one 



through the sub - oesophageal 

 ganglion. The ventral chain is composed of four ganglia con- 

 nected together l^y widely divaricated commissures. Each 

 ganglion gives off three pairs of nerves, two to the ventral mus- 

 culature, and one to the dorsal. The terminations of these 

 nerves in the muscles are very clearly seen in these transparent 

 little creatures, though there is still much dispute as to their 

 exact nature. 



The older writers considered the Tardigrada as hermaphrodites, 

 but Plate and others have conclusively shown that they are 

 bisexual, at any rate in the genus Ifacrobiottis. The males are, 

 however, much rarer than the females. The reproductive organs 

 of both sexes are alike. Both ovary and testis are unpaired 

 structures opening into the intestine, and each is provided with 

 a dorsal accessory gland placed near its orifice. In the ovary 

 many of the eggs are not destined to be fertilised, but serve as 

 nourishment for the more successful ova which survive. 



No special circulatory or respiratory organs exist, and, as in 

 many other simple organisms, there is no connective tissue. 



