258 



ARACHNIDA (INTRODUCTION) 



King-crabs they have become retiform. Mites, Scorpions, and 

 Pedipalps are viviparous, their eggs developing in the ovary or 

 in a uterus. Other Arachnids lay eggs, and many Spiders and 

 Pseudoscorpions carry their eggs about with them. As a rule 

 the young is but a miniature of the parent, and the Arachnid, 

 unlike the Crustacean or Insect, undergoes little or no meta- 

 morphosis. 



A certain number of Mites are parasitic in plants and in 

 animals, and a few, together with a few Spiders, have resumed 

 the aquatic life of their remote ancestors. The members of 

 some Orders, such as the Solifugae and Opiliones, are nocturnal", 

 and many are provided with silk -glands and weave webs which 

 reach their highest pitch of perfection amongst the Spiders. At 

 times especially is this the case with the Mites enormous 

 numbers of individuals live together, but they never show the 

 least adaptation to communal life, and no individuals are ever 

 specialised to perform certain functions, as is the rule in com- 

 munities of social Insects. 



With the two exceptions that we regard the Trilobites as 

 more nearly allied to the Crustacea, and have therefore considered 

 them apart, and have treated the Pycnogonida independently of 

 the Arachnida, we have followed Lankester in his classification, 

 though not always in his nomenclature : 



Sub-class 1. Delobranchiata l (Mero- 

 stomata). 



Order (i.) Xiphosura. 

 Order (ii.) Eurypterida ( = Gig- 

 antostraca, Extinct). 

 Sub-class 2. Embolobranchiata. 



Order (i.) Scorpionidea. 



Order (ii.) Pedipalpi. 



Order (iii.) Araneae. 



Order (iv.) Palpigradi. 



Order (v.) Solifugae. 



Order (vi.) Chernetidea ( = Pseu- 

 doscorpiones). 



Order (vii.) Podogona. 



Order (viii.) Phalangidea ( = Opi- 

 liones). 



Order (ix.) Acarina. 



APPENDICES 



(i.) Tardigrada. 

 (ii.) Pentastomida. 



1 This and the following Sub -class correspond with Lankester's Sub -class 

 Euaraclmida. The Delobranchiata have gills patent and exposed, and adapted for 

 breathing oxygen dissolved in water. The Embolobranchiata have either the gill- 

 books (now termed lung-books) sunk into their body, or the gill-books are wholly 

 or partially replaced by tracheae. In either case the members of this Sub-class 

 breathe atmospheric oxygen. 



