PYCN0G0N01DEA. 



1383 



Affinities of the Arachnoid Entomostraca. — Much doubt has existed 

 with reference to the place of the species in the subkingdom Articu- 

 lata, whether they belong with the Arachnida or Crustacea. Johnston, 

 Edwards, Kroyer, and Quatrefages, arrange them with the Crustacea' 

 though Edwards* observes, that he does it with much hesitation; 

 while nearly all the earlier authors, with Latreille,f and more lately 

 Erichson, have placed them among Arachnida. 



The Articulata are naturally divided into two parallel series one 



consisting of species fitted especially for terrestrial life, and using the 

 air directly in respiration, and the other fitted for aquatic habits, and 

 using the water in respiration. Among the former, there may be 

 aquatic species; but the mode of respiration is still but a slight modi- 

 fication of the general type for the group. So there are terrestrial 

 species in the second division ; but these have the same organs essen- 

 tially as the aquatic, and require moisture in the air in order to carry 

 on their functions of respiration. Crustacea in the second division 

 are parallel with Insects and Arachnida in the first; while Annelida 

 in the second, are analogues of Myriapoda in the first. 



The Pycnogonoidea are those aquatic species that most resemble 

 Arachnida. Yet along with the resemblance, there is the grand distinc- 

 tion which lies between the two sections of Articulata above explained. 

 The mode of respiration is Crustacean; it is aquatic in type, and not 

 merely by adaptation. In Crustacea, respiration takes place by means 

 of the surface of some part of the body, or of its appendages, these 

 parts having thin integuments, so as to allow of the circulating fluid 

 taking air from the water in which the parts are bathed; some species 

 have proper gills, others lamellar appendages to the thoracic or abdo- 

 minal legs, others expose only the surface of the body for this action. 

 Of this last class, are nearly all the Entomostraca, and with them the 

 Pycnogonoids. 



On this ground these species are properly arranged with Crustacea, 

 and among them they have their closest analogies, although pre- 

 senting other relations to the Arachnida. 



Structure. — Kroyer has drawn out an excellent general view of the 

 succession of parts in the species of this group. 

 They are as follows : — 



* Edwards, Crust., i. 230, iii. 530. f Cuvier's Animal Kingdom. 



