90 THE MAIN CURRENTS OF ZOOLOGY 



tacea, lobster, crayfishes, shrimps, sow-bugs, etc. 

 (2) Insecla, an enormous group containing about 

 400,000 described living and 5000 fossil species. 

 Very interesting for exhibiting serial homology of the 

 appendages. These structures, although all equiva- 

 lent, and arising in the same manner, are, under 

 certain conditions, transformed into jaws, antennae, 

 walking-limbs, swimerets, etc. The insects exhibit 

 more clearly than any other animals the phenomona 

 of metamorphosis such as the development of a 

 worm-like larva through the chrysalis stage into the 

 winged insect. These animals are also immensely 

 interesting on account of their relation to the fer- 

 tilization of flowers, their habits, their communal 

 life and as disease carriers. (3) Myriopoda, centi- 

 peds, "thousand-legged worms/ ' etc. (4) Arachnida, 

 or spiders, notable as web-spinners. This class in- 

 cludes scorpions, Limulus (the king-crab) and others. 

 IX. Mollusca. Slugs, snails, clams, oysters, 

 squids, cuttle-fish, devil-fish, etc. Although the name 

 mollusca is from the Latin Mollis, meaning soft, this 

 applies only to the bodies of these animals; a large 

 number of them secrete shells which circumstance 

 makes them of frequent occurrence as fossils. About 

 60,000 living and 21,000 fossil forms are known. 



