310 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



in a fully-developed worm varies from 112 to 130. The 

 worm never exhibits any "secondary" annnlation of the 

 segments, such as occurs so generally in Limnodrilus. 



The Prostomium: 



This appears as an outgrowth or process of the first 

 segment. It is conical in shape with a slightly blunted 

 apex and overhangs the mouth on the dorsal surface. It 

 is separated from the first segment by a transverse furrow. 

 It is extremely sensitive, and is undoubtedly used as an 

 organ of touch, and consequently is plentifully supplied 

 with nerves which arise from the brain. 



Setae: 



The setae, as in all Oligochaeta, are the organs of 

 locomotion. They consist of chitinous rods derived from 

 specialised cells of the epidermis. Part of each seta is 

 buried in the body wall, and may project into the body 

 cavity, but the rest protrudes beyond the surface of the 

 body. 



The development of these organs has been carefully 

 observed and described by earlier writers, especially 

 Vejdovsky, who first established the fact that all setae 

 are ectodermal in origin. A varying number of setae are 

 implanted in " setigerous sacs or follicles" which arise 

 as invaginations of the epidermis. The arrangement of 

 the setae in a sac is like that of the sticks of an open fan 

 (PI. II, fig. 5). Each seta has its origin at the base of the 

 sac, but it is lodged in a separate cavity and divided from 

 its fellows by a small piece of the body wall. The cuticle, 

 which forms the outermost layer of the body wall, is 

 continued into each of the cavities, and forms a lining to it 

 for the greater part of its length. Near the blind end of 

 the sac the cuticle is absent, and is replaced by a large 



