Chap, ix.] SKELETON OF ARTHROPODA. 299 



In this exuviation, or ecdysis, the internal chitinous 

 and calcareous parts are as much affected as the 

 external ; when it is completed the integument of the 

 animal is for a few days soft and moist, but a new 

 exoskeleton is developed with comparative rapidity, 

 while its function as a protective organ is spoken to 

 by the temporary timidity of animals, which, when 

 armed, are bold, and ready to resent attack. 



The integument is smooth in the lower forms 

 only ; in Peripatiis, as in the larvae of insects, 

 it is soft, and in the latter it is sometimes very thin. 

 It is generally stouter in JJJyriopods and Araeli- 

 nida, and its degree of stoutness varies considerably 

 in Insects. Among the Crustacea it is compara- 

 tively soft in Eiatomostraca, except where valves 

 are developed these attain to their greatest hardness 

 in the Cirripedia, which are fixed in the adult 

 stages of their lives, and are therefore unable to 

 escape from enemies ; they are stronger and more 

 compact in the sessile Balanus than in the stalked 

 Lepas ; some Cirripeds have, however, lost their hard 

 valves. In the Malacostracous Crustacea, where the 

 carapace has more definite relations than in such 

 Entomostraca as Apus, knobs,' ridges, or immovable 

 spinous processes, all of which are defensive in 

 function, are very commonly developed. Among 

 the Arachiiida, Limulus is remarkable for the long 

 caudal spine-like termination (Fig. 124) of its shield. 



Internal hard pieces, which appear to have for their 

 chief function that of protecting the central nervous 

 system, whereby they may be compared to the verte- 

 bral column of Vertebrates, are best developed in the 

 higher Crustacea. Though topographically and func- 

 tionally internal, these parts are morphologically ex- 

 ternal, and they share in the general moulting of the 

 tegumeiitary skeletal parts ; these ingrowths are 

 known as the apo denies. In the crayfish, for 



