336 



ARACHN01DEA AND PALMOSTRACA. 



The limbs seem to be more like those of Crustaceans than those of 



Arachnoids, and the 

 occurrence of antennae, 

 observed by Linnseus 

 (1759), and recently cor- 

 roborated, accentuates 

 the resemblance. The 

 affinities with Limtdus, 

 according to the views 

 of other authorities, 

 justify the association 

 of Trilobites and Arach- 

 noids. A compromise 

 may be perhaps effected 

 by regarding the Trilo- 

 bites as an offshoot from 

 a stock ancestral to 



Fig 



, 146. — Vertical cross-section of a Trilobite 

 (Calymene). — After Walcott. 



Intestine ; s. } shield ; Z., endopodite ; c, exo- 

 podite ; b. t epipodial parts. 



Incerice Sedis. 



both Arachnoids 

 Crustaceans. 



and 



Pantopoda or Pycnogonimj. 



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 may 

 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 chelicene when these are present, 

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

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



Examples. — Pycnogonum, Nymphon, Ammothea. 



Fig. 147. — Sea-spider {Pycnogonum 

 Htlorale), from the dorsal surface. 



The first two pairs of t appendages are ab- 

 sent. In the anterior region are four 

 simple eyes. 



