PHYLUM ARTHROPOD A 231 



The pycnogonids rather closely resemble the spiders, but the 

 presence of seven pairs of appendages is a character not encoun- 

 tered in the Arachnida. There is considerable question as to 

 the proper place to include the members of this aberrant group. 

 Nymphon (Fig. 110) is a characteristic genus. 



Class 4. Tardigrada 



The water-bears or tardigrades are microscopic organisms 

 living in both fresh and salt water. The body is provided with 

 four pairs of unsegmented appendages each of which bears 

 terminal claws. The number and form of these claws differ 

 in the various genera. Neither antennae nor mouth parts are 

 found on the head. Thus far neither respiratory nor circulatory 

 organs have been demonstrated. 



A single gonad opens into the cloaca. The nervous system 

 consists of a brain, a subesophageal ganglion, and a ventral 

 chain of four ganglia. 



There is a possibility that the tardigrades may belong to 

 the annelid group, though the internal organization seems to 

 indicate degenerate arthropod relationships. Macrobiotus is 

 the name of one genus. 



Class 5. Onycophora 



The genus Peripatus and several other genera closely allied 

 to it present such a mixture of annelidan and arthropodan 

 characters that much significance is usually attached to these 

 forms as a possible connecting link between the worms and the 

 arthropods. The various species, which have been encountered 

 in Africa, Australia, New Zealand, Central and South America, 

 and the West Indies, comprise several genera which together 

 seem to warrant their being grouped as an independent class. 



The body is wormlike or caterpillar-like in form, without exter- 

 nal marks of segmentation but with numerous paired legs the 

 number of which varies in different species. Though these legs 

 have a ringed appearance, they are not distinctly jointed, and in 

 this respect they seem to be intermediate between the parapodia 

 of worms and the jointed legs of arthropods. The metameric 

 arrangement of the legs is paralleled by some features of the 

 internal organization. Near each pair of legs, the ventral nerve 

 cords bear a slight enlargement, though ganglia are not differen- 

 tiated. The base of each leg carries a nephridial opening. Thus 



