304 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



and are supplied with well-developed muscles. In 

 the aquatic forms, such as the water-beetles (Dyticus), 

 the coxse of the third pair of legs are flattened and 

 oar-like. Such as float on the surface of the water 

 have the contained air tubes enlarged to serve as float 

 bladders, or the legs are greatly elongated so as to 

 extend over a large surface of water. In climbing 

 insects the claws may be cleft or pectinated so as to 

 enable them to hold on to small objects ; or an at- 

 taching lobule may be developed between the claws. 



The tergal portions of several successive seg- 

 ments may unite with one another, and thus give 

 greater firmness to the dorsal surface ; this process 

 may result in the formation of a free-projecting shell, 

 as in Apus, or this shell may become divisible into 

 two valves, as in the Ostracoda, or the fusion may 

 extend far back, as in the crayfish, or the scorpion, 

 where we have the so-called cephalo thoracic cara- 

 pace ; in Limulus the sides of the carapace are pro- 

 duced, and we get the well-known large shield of 

 these animals ; the same phenomenon in the crayfish 

 or the lobster results in the formation of a special 

 wall for the branchial chamber. 



The most remarkable modifications are exhibited 

 by the Cirripedia, where the exoskeleton is ordi- 

 narily in the form of calcified valves, two on either 

 side of the body, and in Lepas with a dorsal median 

 piece ; in Balanus these valves are withdrawn into 

 a special shell. 



In the Mollusca the characteristic organ of sup- 

 port and defence is an external calcareous shell, 

 which is formed by the mantle, and which, when 

 aided by the operculum, which is developed on the 

 base of the foot, becomes so completely an organ of 

 protection that many snails hibernate in their closed 

 shell ; the tenant of an exotic shell has been 

 bought, sold, and exhibited in a museum for the space 



