424 



CHARACTERS OF INVERTEBRATE ANIMALS 



Class 7.— SEA-SPIDERS (Pycnogonida) 



These are widely distributed marine Arthropods, mostly of 

 small size. Some zoologists associate them with Arachnida, but 



their real affinities are as yet un- 

 determined. Taking such a typical 

 form as the Shore Pycnogon {Pycno- 

 gonum littorale), we see (fig. 263) 

 that the spider -like appearance is 

 due to the presence of four slender 

 pairs of thoracic limbs, in front of 

 which are three pairs of smaller 

 appendages, of which the first bear pincers and the last are used 

 by the males for carrying the eggs about. The mouth is situated 

 on the end of an elongated snout, and there are four simple eyes. 



Fig. 263. — Shore Pycnogon {Pycnogonnm littoral^ 



A, From above : J, from below, ab. Abdomen ; 

 ov. egg-bearing appendages; PR. snout. 



