98 Invertebrata. 



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

 Myriopoda, centipedes, &c., 4. Jnse:ta. 



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. 



From two to six of the 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 supraoesophageal nerve-ganglion or 

 brain, which sometimes is large and complex, as in ants. 



CHAPTER XIX. 



CRABS AND LOBSTERS. 



CLASS I. Crustacea. The animals of this class are 

 all water-breathers, either provided with gills, or else 

 with a thin integument through which the blood 

 becomes directly aerated. The structure of a crustacean 

 can be easily understood by examining a lobster or 

 freshwater crayfish. In either of these we notice that 

 the body is divided into two regions, an anterior, 

 covered by a dorsal shield of two pieces, and a 

 posterior, consisting of a series of rings, ending in 

 the fan-like tail. In the body of the lobster we notice 



