OS Invertebrata. 



2. Arachnoidea, spiders, mites, and scorpions ; 3. 

 Myriopoda, centipedes, &c.; 4. Insesta. 



Many arthropods are parasitic, and these are at 

 first not unlike allied non-parasitic species, but 

 shortly after hatching they retrogress, such parts as 

 are not necessary disappear and hence the adult 

 parasites are in their organisation much simpler even 

 than they themselves were in their embryonic states ; 

 but as has been already noticed, the egg-producing 

 organs are much increased in development. 



The two, four or six foremost segments of the 

 body in arthropods become united to make up a head, 

 which carries sentient organs, such as the eyes, ears, 

 and antennae or feelers, with the mouth. In the head 

 likewise is the large supra-cesophageal nerve-ganglion 

 or brain, which sometimes is large and complex, as in 

 ants. 



CHAPTER XIX. 



CRABS AND LOBSTERS. 



CLASS T. Crustacea, The animals of this class are 

 all water-breathers, usually with a calcified integument, 

 hence the name, either provided with gills, or else with 

 a thin surface layer through which the blood becomes 

 directly aerated. The structure of a crustacean can 

 be most easily learned by examining a lobster or fresh- 

 water crayfish. In either of these the body is divided 

 into two regions, an anterior, covered by a dorsal shield 

 of two pieces one in front called ce> halostegite, and a 

 hinder called omostegitesm& a posterior, consisting of 

 a series of rings, ending in the fanlike tail. There are 



