Post- Larval Stages of Arenicola 81 



the closed statocysts, each with a single, spherical, secreted statolith. 

 The following segment is achaetous in all the examples seen by the 

 writer, but Profs. Mesnil and Fauvel have recorded the presence, in 

 some^of their specimens, of a single capillary chaeta in this segment. 

 The succeeding segments, except in some cases the two or three last 

 formed, are chaetiferous. In several of the posterior notopodia there 

 is for a short time a crotchet, accompanied usually by one or more 

 capillary chaetae (p. 40). The characters of the chaetae are described 

 on pp. 40, 43, 53. 



Gills are present only in well-grown examples, not less than about 

 8 mm. long, with approximately sixty fully formed chaetiferous 

 segments. The gills first appear on the sixteenth to eighteenth 

 segments, and then on the succeeding segments (PI. XI, Fig. 35) ; but 

 for a considerable time the posterior segments are abranchiate. For 

 instance, specimens with sixty-two and sixty-three segments are gill- 

 less behind the thirty-second and forty-fourth respectively. By the 

 time the gills have become bifid or trifid the worm begins to change 

 its habitat. It leaves the bases of the algae among which it has 

 hitherto lived, and moves to sand or gravel, in which it commences 

 to burrow. 



The body- wall is reddish or pale -greenish yellow, or it is dark 

 green, owing to the presence of a considerable amount of pigment, 

 which is especially abundant in the first few segments. 



The alimentary canal has assumed the adult form, and the 

 iiephridia, of which there are thirteen pairs, are already (in specimens 

 10 mm. long) becoming saccular, and a definite funnel is present ; 

 but the dorsal lip has not yet developed the processes present 

 thereon in the adult. 



Post-Larval Stages of AKENICOLA BRANCHIALIS. 



Among a collection of post-larval and young stages of Arenicola, 

 taken in September, 1910, among the "roots" of Laminaria in 

 Blacksod Bay (Mayo), by Mr. E. Southern, the writer has found four 

 post-larval examples of A. hranchialis, the first known specimens. 

 They are 4 '4, 5*5, 5 '8 and 6'5 mm. long, and have thirty-eight, 

 forty-two, forty and forty-one chaetiferous segments respectively. In 

 the first, third and fourth specimens there are two newly-formed 

 achaetous segments immediately in front of the pygidium. The 

 specimens are abranchiate, and exhibit yellow pigmentation anteriorly 

 and posteriorly. 



G 



