322 Shield- bearing Crustacea. 



the larval state the worm-like caterpillar or grub precedes the 

 imago. 



In the Crustacea almost as great a diversity exists between 

 the highest and lowest orders as between the worm and the 

 butterfly ; but the larval stages of the higher forms are found 

 closely to simulate the adult state of some of the lower, and 

 seem to suggest that the latter are only the arrested stages of 

 the former — with powers of reproduction added. 



In the Mammalia (the highest form of Vertebrata) the bony 

 framework is internal, being clothed with muscles, to which it 

 gives firmness, and furnishes points of attachment and fulcra. 



This is a living framework, moreover, and is kept in repair 

 and supplied with blood-vessels which nourish it ; for it has to 

 last the lifetime of the individual. 



In the Crustacea (which may serve as an example of the 

 Invertebrata) , on the contrary, the framework is external, the 

 dermal cuticle being hardened by the deposition of calcareous 

 matter and chitine, and within this, as in a coat of mail, the 

 soft organs are placed, and to it the muscles of the animal are 

 attached. 



This is not, however, a permanent, but a deciduous skele- 

 ton, being cast off and reproduced as often as the growth of 

 the creature necessitates its increased size. 



These two forms, the Mammalian and Crustacean, are types 

 of the endo and exo-skeleton (the internal and external). 



In the endo-skeleton the nervous centre is placed upon the 

 dorsal surface, and the nutritive system on the ventral ; but in 

 the exo-skeleton the reverse is the case, the viscera being 

 placed along the back, and the nervous ganglia down the 

 ventral surface. 



Again, in the Vertebrata the principal development of 

 sentient power is concentrated to form the brain, which is the 

 capital of the whole nervous system : in the Annulosa, how- 

 ever, each separate ring or segment of the body has its own 

 ganglion or nerve-mass, and each ganglion, in the simpler 

 forms, seems equal to every other, and there is no specializa- 

 tion of any. But this simplicity of structure gives way to 

 more complex forms as we ascend in the animal scale. 



In insects the body is said to be composed of thirteen 

 rings — head, one; thorax, three; abdomen, nine; but ento- 

 mologists know that the head in insects is in reality made up 

 of at least four segments blended together ; for it is a funda- 

 mental rule in the Annulosa, that every body-ring bears one 

 pair of appendages, and only one pair. Therefore, if we find 

 an insect's head, bearing eyes, antennae, maxillae, and mandibles, 

 we shall be quite safe in concluding that it is made up of 

 several segments soldered together. 



