6 F. W. GAMBLE AND J. H. ASHWOETH. 



belong, thus confirnnng Benliam's observations on the post- 

 larval stage (1893). 



The anterior region of the body is thus composed of the 

 prostomium, six chsetigerous somites, and a region between 

 these, made up probably of two somites, but the exact number 

 is somewhat doubtful. (See Plate 1, fig. 1, and explanation, 

 p. 39.) 



The second or branchial region of the body is composed of 

 thirteen somites, and is distinguished by the presence of gills, 

 a pair of which are attached to a slight fold of the skin just 

 behind the notopodia. The first gill is variable, usually fairly 

 well developed, but always smaller than the rest and sometimes 

 absent. The gills about the middle of the branchial region 

 are frequently, but not always, the largest. Both the gills 

 and notopodia are very sensitive, and are retracted from time 

 to time on the application of stimuli, such as a strong light. 

 This contraction of the gills proceeds sometimes as a wave 

 down the body, and as Milne Edwards (1838) pointed out in 

 his classical paper, considerably assists the circulation of the 

 blood. The neuropodia in the branchial region extend towards 

 the mid-ventral line, so as almost to meet, and are only 

 separated by a groove which marks the line of the nerve-cord. 

 This groove is continued on to the prostomium by a pair of 

 diverging arms {" Metastomial grooves ") underlying the cir- 

 cum-oesophageal nerve connectives (PI. 4, fig. 19, C. Mt.). 



The tail, which is devoid of setse and gills, is marked by a 

 large number of secondary annuli, crowded together at first, 

 but arranged in distinct somites of about five each, towards the 

 hinder end. The caudal region varies much in length; some 

 specimens have about thirty somites, but the number is not 

 constant, possibly owing to the tendency of the worm to throw 

 off the last few segments when irritated. 



There is no change in the internal organs to mark tlie 

 somite which bears the first gill, but the transition from the 

 branchial to the caudal region is accompanied by the loss of 

 parapodia, oblique muscles, and branchial vessels. 



Jixternal apertures. — The mouth (PI. 4, fig. 19, C.MQ.), 



