Backboneless Animals 



239 



The members of the last four classes usually breathe by means 

 of air-tubes or tracheae, which penetrate into every part of the body, 

 or in" the case of spiders and scorpions, by "lung-books," which 

 seem like concentrated and plaited tracheae. The King-crab 

 (Limulus), which is very often ranked along with Arachnids, is 

 aquatic, and breathes by peculiar "gill-books." 



(«) Crustacea. — Except the wood-lice, which live under bark 

 and stones, the land-crabs which visit the sea only at the breeding 



Fig. 45.— Nauplius of Sacculina. (From Fritz Miiller.) 



time, and some shore-forms which live in great part above the tide- 

 mark, the Crustaceans are aquatic animals, and usually breathe by 

 gills. Each segment of the body usually bears a. pair of append- 

 ages, and each appendage is typically double. Among these ap- 

 pendages much division of labour is often exhibited, some being 

 sensory, others masticatory, others locomotor. In the higher forms 

 the life-history is often long and circuitous, with a succession of 

 larval stages. 



The lower Crustaceans are grouped together as Entomostraca. 

 They are often small and simple in structure ; the number of 



