232 The Study of Animal Life part m 



earthworms (Lumbricus, etc.), in the freshwater worms (Nais, 

 Tubifex, etc.), in the lobworms {Arenicola piscatoruni), and in the 

 sea-worms {Nereis, Aphrodite, etc.), all of which are ranked as 

 Chsetopods, 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 feet 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 united 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 lower 

 "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 concentrated 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 and a ventral chain of ganglia — is also 

 characteristic of crustaceans, insects, and related forms. 



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 feet ; 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 feet, and eyes and tentacles 

 and gills, but there is much difference between those which swim 

 freely in the sea (e.g. Alciope and Tomopteris and some Nereids) 

 and the lobworms which burrow and make countless castings upon 

 the flat sandy shores, or those which inhabit tubes of lime or 

 sandy particles (e.g. Serpula, Spirorbis, and Lattice or Terebella 

 conchilegd). The earthworms with comparatively few bristles 

 (Oligochaeta) are bisexual, while almost all the marine worms with 

 many bristles (Polychaeta) have separate sexes. Moreover, those of 

 the first series usually lay their eggs in cocoons, within which the 

 embryos develop without any metamorphosis, while the sea-worms, 

 though they sometimes form cocoons, have free-swimming larvae 



