258 ARACHNIDA (INTRODUCTION) CHAP. IX 
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! (Mero- Order (vi.) Chernetidea (= Pseu- 
stomata). doscorpiones). 
Order (i.) Xiphosura. Order (vii.) Podogona. 
Order (ii.) Eurypterida ( = Gig- Order (viii.) Phalangidea (= Opi- 
antostraca, Extinct). licnes). 
Sub-class 2. Embolobranchiata. Order (ix.) Acarina, 
Order (i.) Scorpionidea. 
Order (ii.) Pedipalpi. | APPENDICES 
Order (iii.) Araneae. (i.) Tardigrada. 
Order (iv.) Palpigradi. (ii.) Pentastomida. 
Order (v.) Solifugae. 
1 This and the following Sub-class correspond with Lankester’s Sub-class 
Kuarachnida. 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. 
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