ANNELIDA. 239 
and most primitive of the Annelida, as is shown by the 
ectodermal nervous system, the persistence of radial septa 
and longitudinal mesenteries, the simple nephridia and the 
absence of appendages. It contains two or three other small 
worms. 
Cxass II.—Potycuata. This class has a great number 
and variety of types. Many live in tubes and burrows and 
the anterior end bears a mass of tentacles and gills, whilst 
the free-swimming forms often have a great development 
of lateral appendages which are in many cases used for 
swimming. They are called Polychaeta because they usually 
have great numbers of setze. 
Fig. 157.—FooT oR PARAPODIUM OF A NEREIS. (Ad nat.) 
Dorsal Cirrus. 
Notopodium. 
Acicula. 
Ventral Cirrus. Neuropodium. 
Crass III.—Ouicocuata. As in Lumdbricus, the body 
is usually without appendages or gills and has only com- 
paratively few sete. They are usually divided into the 
mud dwelling (freshwater) forms and the terrestrial. Their 
hermaphrodite and complex sexual organs and protected 
embryonic development are characteristics. 
Crass 1V.—HirupinzEa. In many respects this class 
resembles the last, especially in the absence of appendages, 
the hermaphrodite sexual organs and the development. 
It is, however, clearly characterised by the reduced con- 
dition of the coelom and its continuity with the blood- 
vascular system, by the suckers and the mode of life. 
The most important features of the sub-phylum and the 
classes are summarised in the subjoined table :— 
