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- 

 cucumbers, 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 de 

 velopment 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 shape ^ 

 assumed by animals of 

 varied nature who began 

 to move head foremost 

 and to acquire sides. 

 There is no class of 

 " worms," but an as- 

 semblage — a mob — 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 twQ 

 chief classes ; — ^ 



Fig. 13. — Crinoid or feather-star. 



