10 GENERAL SURVEY OF THE ANIMAL KINGDOM. 
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 
Fic. 12.—Spider. 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_ shape, 
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 Fic. 13.—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 :— 
