858 CLASS V. ARACHNIDA. 



There are no special organs of circulation or of respiration. 

 The corpusculated fluid in the body-cavity is moved about 

 intermittently by the movements of the legs, teeth, etc., and 

 this fluid and the tissues must be oxygenated through the thin 

 cuticle. 



The nervous system comprises a supraoesophageal ganglion r 

 an infraoesophageal ganglion, and a ventral ladder of four 

 ganglia. The supra-oesophageal ganglion is somewhat rounded ; 

 posteriorly it is produced on each side into two lobes, one of 

 which bears an eye ; close to this a slender nerve emerges which 

 runs straight to the first ventral ganglion. The large mass of 

 the brain is curved round the oral cavity and enters the sub- 

 oesophageal ganglion. This is united to the first ventral ganglion, 

 and this in its turn to the next, and so on, by widely divaricated 

 longitudinal commissures. The transparency of the tissues 

 and the absence of connective tissue allow the nerves supplying 

 the curiously symmetrical muscles to be readily traced to their 

 nerve-plate or nerve-end-plate. 



There has been much confusion about the sex of the Tardi- 

 grada. For a long time they were thought to be hermaphrodite, 

 but more recently the genus Macrobiotus has been shown to be 

 bisexual, and probably the same holds good for all the genera. 

 The males are smaller than the females and much rarer. The 

 testes and ovary open into the intestine near the posterior end 

 and each is provided with a dorsally placed accessory gland. 



Only some of the eggs develop, and this they do at the expense 

 of the others. The segmentation of the egg of M. macronyx is 

 total and equal, and a typical blastula and gastrula are formed. 

 The blastopore closes and the anus ultimately arises at the same 

 spot. Four pair of mesoblastic somites are formed and give 

 origin to the coelom and to the musculature. Botli the 

 malpighian tubules and the testes and ovary arise as diverticula 

 of the alimentary canal. 



The details of the development seem to be primitive in their 

 nature, and afford no support to the view that the Tardigrada 

 are in any way degenerate. They may be regarded as the 

 simplest and lowest group of Arthropods, but they show little 

 affinity to any of the now existing forms. The fact that different 

 authorities have placed the Tardigrada near the Crustacea, the 

 Rotifers, the Annelids, the Acarina, the Pentastomida, and 



