CILETOPODA 295 



the body cavity, and find their way to the exterior either by 

 dehiscence of the body walls, or through the nephridial tubes 

 (so-called segmental organs). 



The Polychseta fall naturally into two main divisions, 

 the free and the sessile forms, or Errantia and Sedentaria. 

 In many of the Errantia there are no sexual differences, or 

 indeed any accessory structures related to reproduction. 

 But in certain families, especially the Nereidae and Syllidse, 

 very remarkable transformations and differentiations are 

 connected with the reproductive function. These changes 

 of form are typically presented in the genus Nereis. In 

 the ordinary form of this genus there are a number of 

 segments all similar, with the exception of a few at the 

 anterior and posterior extremities, and all provided with 

 pairs of similar parapodia. Some species, such as the 

 British form N. diversicolor, undergo no change when 

 sexually mature. In other species the posterior segments, 

 throughout more than half of the body length, acquire, in 

 sexually ripe individuals, a different character. Such indi- 

 viduals, consisting of two conspicuously different body- 

 regions, were at one time supposed to belong to a distinct 

 genus, and were described in detail as species of Heteronereis. 

 It is now known, however, that the condition is produced 

 by the transformation of individuals of the ordinary Nereis 

 form. The change is due (a) to an alteration in the shape 

 and size of the parapodia in the posterior part of the body, 

 and (b) to a difference in the form and number of the chsetae 

 of these parapodia. With regard to (a), the various lobes 

 of the parapodia are much enlarged and flattened antero- 

 posteriorly, while (b) the old chaetse are shed and replaced by 

 new, of a different shape, the blade being flattened and 

 paddle-shaped. Both these changes render the parapodia 

 more efficient as organs of propulsion through the water, 

 and as a matter of fact the worms in the Heteronereis 



