ANNELIDA 261 



by it. They retain the segmentation characteristic of the phylum in most 

 of their organs but the coelom is usually much restricted and broken up 

 into a system of small spaces and the chaetae are lost. In one primitive 

 form, Acanthobdella, there are chaetae and a spacious perivisceral 

 coelom in the anterior segments. In all leeches the anterior and 

 posterior suckers and a hermaphrodite reproductive system, closely 

 paralleled in a subdivision of the Chaetopoda, the Oligochaeta, show 

 the speciahzation of the group. The Echiuroidea and Sipunculoidea 

 are two groups of burrowing marine worms in which segmentation 

 has been almost entirely suppressed in the adult but is sometimes 

 shown in the larvae by mesoblastic somites or ganglion rudiments. 

 Chaetae are lost except in a few forms, but a large perivisceral coelom 

 is preserved. 



Class CHAETOPODA 



Well-segmented Annelida, with chaetae and a spacious perivisceral 

 coelom, usually divided by intersegmental septa. 



In a typical chaetopod there is a distinct preoral region or pro- 

 stomium and a postoral body composed of many segments. Each 

 segment owes its distinctness to the development in the larva of a 

 pair of mesoblastic somites which join round the gut, the cavities 

 which develop in them becoming the perivisceral cavity of the adult 

 segment. At the same time the larval ectoderm (epiblast) develops 

 segmentally repeated organs : the ganglia^ swellings in the continuous 

 ventral nerve cords, the nephridia or excretory organs and the chaetae. 

 In the Polychaeta, one of two orders into which the Chaetopoda are 

 divided, the chaetae are borne in groups upon processes known as 

 parapodia, whose projection from the body wall is due to the develop-, 

 ment of special muscles for moving the chaetae. In the other order, 

 the Oligochaeta, there are no parapodia. The chief feature of the 

 nervous organization is that the musculature of all parts of the body is 

 co-ordinated by metamerically repeated intra- and intersegmental re- 

 flexes (Fig. 183). In each segment there is, for example, a correlation 

 of the circular and longitudinal muscles by the segmental nerves which 

 acts so that contraction of one brings about automatically relaxation of 

 the other. Then there are nervous connections between adjacent 

 segments which act so that excitation of a muscle layer in one segment 

 leads to excitation of the same layer in the other segment. By the 

 working together of the inter- and intra-segmental reflexes the normal 

 peristaltic movement of Lumbricus and other chaetopods is brought 

 about. 



There is also a system of giant fibres, three in number, running 

 along the whole length of the ventral nerve cord. These are responsible 

 for the reactions which require immediate co-ordination of the whole 



