PANTOPODA OR PYCNOGONIDA. 



363 



corroborated, accentuates the resemblance. The affinities with 

 Litnuhis, according to the views of other authorities, justify the 

 association of Trilobites and Arachnoids. A compromise may be 

 perhaps effected by regarding the Trilobites as an offshoot from a stock 

 ancestral to both Arachnoids and Crustaceans. 



Incerta; Sedis. 



Class PANTOPODA or PYCNOGONIDA. 



These are marine Arthropods, sometimes called sea-spiders. Their 

 affinities are uncertain, but perhaps they may be ranked between 

 Crustaceans and Arachnoids. Many climb about sea-weeds and hydroids 

 near the shore, but some live at 

 great depths. The body con- 

 sists of an anterior proboscis, 

 a cephalothoracic region with 

 three fused and three free seg- 

 ments, and an unsegmented 

 rudimentary abdomen. There 

 are typically seven pairs of ap- 

 pendages. Of these the first 

 are short and chelate, but may 

 be absent in the adult. The 

 next two are small and slender, 

 and are often absent in the adult 

 female ; the second pair m ay 

 also be absent in the male, but 

 the third in the males of all 

 genera carries the eggs. The 

 last four pairs of appendages 

 are always present, and form 

 the exceedingly long walking 



legs. Into them, and into the chelicerse when these are present, 

 outgrowths of the mid-gut extend. The sexes are separate. The 

 larvre are at first unsegmented, with three pairs of appendages. 



Examples. Pycnogoniun, Nymphon, Ammothea. In Pentanymphon 

 and Decolopoda there is an extra pair of long walking legs. 



FIG. 180. Sea-spider {Pycnogonum 



littorale], from the dorsal surface. 



The first two pairs of appendages are ab- 

 sent. In the anterior region are four 

 simple eyes. 



