264 



PHYLUM ANNELIDA 



fashion in the httle worm Polygordius. We shall therefore follow it 

 there (Fig. 143). 



In the larva, which is a typical Trochosphere, the first sign of 

 segmentation appears in the mesoderm bands. These arise by division 

 of the primary mesoderm cells of the embryo, which form two colmnns 

 of cells extending downwards. At the same time the posterior region 

 of the larva elongates greatly, carrying the larva hind-gnt with it. 

 Externally the growing body is marked by constrictions, internally 

 by mesoderm cavities. These cavities, taken together, form the adult 

 body cavity ; the outer and inner walls form the somatic and splanchnic 

 layers ; the posterior and anterior walls of adjacent segments fuse to 

 form the septa of the adult worm ; the inner (splanchnic) walls of the 

 primitive segments on each side fuse above and below the gut to form 

 the dorsal and ventral supporting mesenteries of the gut. The head 

 region is at first disproportionately large, but later, by an independent 

 process of growth, becomes reduced. The larva abandons its pelagic 

 life, and becomes adult. 



Comparing the development of Polychasta with this, we find that the 

 Trochosphere is often modified, and that segmentation tends constantly 

 to appear at an earlier stage. As a further step in the same direction, 

 we may note that in some Polych»ta the Trochosphere stage is no 

 longer recognisable as such. 



The two chief orders of this class may be contrasted : — 



General Survey of the Class Ch^topoda 



I. Oligochaeta. — The general characters may be gathered from 

 the description of the earthworm, but it is to be noticed that the earth- 

 worms are specialised forms, and that the fresh-water Oligochaetes are 

 of much simpler structure. The most essential distinction from the 

 Polychaeta is to be found in the complex reproductive organs. The 

 absence of gills, though general, is not universal, for a few fresh-water 

 forms, such as Dero and Branchiura, possess gills of simple structure, 

 while the West African Alma niloiica has more complex liranched 

 retractile gills. Among other characters may be noticed the tendency 



