xii BACKBONELESS ANIMALS 239 



tor urn), and in the sea-worms (Nereis, Aphrodite, etc.), 

 all of which are ranked as Chastopods, the body is divided 

 into a series of similar rings or segments, and there are 

 always some, and often very many, bristles on the outer 

 surface. The segments are not mere external rings, 

 but divisions of the body often partially partitioned off 

 internally, and there is usually some repetition of internal 

 organs. Thus in each segment there are often two little 

 kidney-tubes or nephridia, while reproductive organs 

 may occur in segment after segment. Moreover, there 

 are often two unjointed appendages or parapcdia on each 

 ring. The nervous system consists of a dorsal brain and 

 of a double nerve-cord lying along the ventral surface. 

 The nerve-cord has in each segment a pair of nerve-centres 

 or ganglia, and divides in the head region to form a ring 

 round the gullet which unites with the brain above. The 

 existence of nerve-centres for each segment makes each 

 ring to some extent independent, but the brain rules all. 

 This type of nervous system represents a great step of 

 progress ; it is very different from that of Stinging-animals, 

 which lies diffusely in the skin or forms a ring around the 

 circumference ; different from that of the low r er " worms," 

 where the nerve-cords from the brain usually run along 

 the sides of the body ; different from that of molluscs, 

 where the nerve-centres are fewer and tend to be con- 

 centrated in the head ; different finally from the central 

 nervous system of backboned animals, for that is wholly 

 dorsal. But the type characteristic of ringed ' worms ' 

 -a dorsal brain, a ventral chain of ganglia and a ring 

 round the pharynx or gullet connecting the two is also 

 characteristic of crustaceans, insects, and related forms. 

 It is called the Annulate type of nervous system. 



Of bristle-footed " worms," there are two great sets, 

 the earth-worms and the sea- worms. The former, 

 including the common soil-makers and a few giants, such 

 as the Tasmanian Megascolides, sometimes about six feet 

 long, have bristles but no parapodia ; sense-organs, 

 feelers, and breathing organs are undeveloped, as one 

 would expect in subterranean animals. The sea-worms, 

 on the other hand, have usually stump-like bristly para- 



