10 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



FIG. 12. Spider. 



appendages. The skin produces an external, not-living 

 cuticle, the organic part of which is a substance called 

 chitin, associated in Crustaceans with carbonate of 

 lime. The nervous system con- 

 sists of a dorsal brain, connected, 

 by a nerve -ring around the 

 gullet, with a ventral chain of 

 ganglia. 



Echinoderms. This is a well- 

 defined series, including star-fishes, 

 brittle-stars, sea-urchins, sea-cucum- 

 bers, and feather - stars. The 

 symmetry of the adult is usually 

 radial, though that of the larva is 

 bilateral. A peculiar system, known 

 as the water - vascular system, is 



characteristic, and is turned to various uses, as in 

 locomotion and respiration. There is a marked tend- 

 ency to deposition of lime in the tissues. The develop- 

 ment is strangely circuitous or "indirect." 



Segmented "worms." 

 It is hopeless at 

 present to arrange with 

 any definiteness those 

 heterogeneous forms to 

 which the title "worm" 

 is given. For this title 

 is little more than a 

 name for a shapc> 

 assumed by animals of 

 varied nature who be- 

 gan to move head 

 foremost and to acquire 

 sides. There is no 

 class of "worms," but 



an assemblage a mob JT IG> I3 ._ Crinoid or feather-star. 

 not yet reduced to 



order. It seems useful, however, to separate those which 

 are ringed or segmented from those which are unsegmented. 

 The former are often called Annelids, and include two chief 

 classes : 



