ARACHNIDA 467 



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 littorale, is found 

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

 anemones in 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 men- 

 tioned 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 they 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. 261). 



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. Macrohiotus (Fig. 346 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 

 tardigrades have been compared to the tortoises among the verte- 

 brates, from their slow and awkward gait. The mouth opens into 

 a tube in which work the two chitinous stylets; a suctorial pharynx, 

 the wall of which is composed of radiating muscular fibres, follows. 

 Into the pharynx opens a pair of salivary glands. The animals pierce 

 the wall of plant cells with the stylets and suck the sap by the action 

 of the pharynx. Then comes a narrow oesophagus leading into a 



30-2 



