ARACHNIDA 539 



palps, the ovigerous legs which are present in both sexes and the first 

 pair of walking legs; (3) three free segments bearing the remaining 

 pairs of walking legs. The body is usually very small while the legs 

 are enormously elongated. They have eight joints. The proboscis con- 

 tains a sucking pharynx preceded by a filter of chitinous hairs which 

 prevents any but fluid food from proceeding further. The small 

 stomach gives off" digestive coeca which extend into the legs and other 

 appendages. The common British form, Pycnogonum Itttorale, is found 

 firmly attached by the terminal claws of the legs to the sides of sea 

 anemones into which it inserts the proboscis and sucks the juices. There 

 is a dorsal heart with three pairs of ostia; respiration is cutaneous. 

 The nervous system consists of supraoesophageal ganglia and a 

 ventral chain with suboesophageal and three or four other ganglia. 



The sexes are separate and the males carry the eggs on the oviger- 

 ous legs. The gonads, like the alimentary canal, are branched and open 

 on the 4th segment of the legs (the last pair of legs in Pycnogonum or 

 all four pairs in Phoxichilidium femoratum). In the latter species the 

 larvae are hatched as six-legged creatures, which form cysts in the 

 polyps of the gymnoblast hydroid, Coryne. 



Four small classes, Pseudoscorpionidea, Pedipalpi, Solifugae and 

 Palpigradi, are undoubtedly arachnids, but can merely be mentioned 

 here. 



The two small classes following have been associated with the 

 arachnids but no sufficient reason can be advanced for this. They 

 both exhibit simplicity of structure ; in the case of the Pentastomida 

 this is due to parasitism, but in the Tardigrada some of the traits of 

 primitive arthropods may be preserved. In some ways the Tardigrada 

 resemble Peripatus and their development is said to be of a very 

 primitive type. But the size and specialized habitat incline the author 

 to regard this as a case of "simplification" such as is met with in the 

 Archiannelida (p. 294). 



Class TARDIGRADA 



Minute arthropods with four pairs of stumpy legs ending in claws, 

 with oral stylets and a suctorial pharynx, without definite circulatory 

 or respiratory systems. 



Representatives of this group, e.g. Macrobiotus (Fig. 370 B), are 

 found, for instance, in moss and in the sediment of rain gutters. They 

 are minute and often very transparent animals, with a thin and 

 flexible cuticle. The body is usually short and flattened; the tardi- 

 grades have been compared to the tortoises among the vertebrates, 

 from their slow and awkward gait. The mouth opens into a tube in 



