ANATOMY AND OLASSIFIOATION OF THB ARBNICOLID.E. 525 



are exceedingly small but just recognisable in a specimen 

 44 mm. long. 



In full-grown specimens of A. marina the gonad on the 

 last five pairs of nepliridia is a somewhat elongated, club- 

 sliaped, or cylindrical body, from •4. mm. to 1 mm. in length, 

 surrounding the gouidial vessel immediately after it leaves 

 the funnel. Cosmovici (1880) says that there are six pairs of 

 gonads, but in all our specimens there are only five, the first 

 nepliridia not bearing gonads. 



In A. Claparedii the gonad which is present only on the 

 last four pairs of nepliridia occupies a similar position to 

 that of A. marina. It is ovoid or club-shaped, and about 

 •4 mm. to '9 mm. long (PI. 24, fig. 29). 



As in the two preceding species, the first nephridium of 

 A. cr is tat a bears no gonad, but the gonidial vessel is 

 present. The gonidial vessel of the other five pairs of 

 nephridia is thinly covered by genital tissue from the point 

 at which it leaves the posterior portion of the nephrostome 

 almost until it reaches the vesicle, a distance of 5 mm. to 

 7 mm. in the specimen shown in PL 24, fig. 30. 



The gonadsof A. Grubii appear early,being already present 

 on the last four pairs of nephridia of a specimen 35 mm. long. 

 At this stage the gonad consists of a small cylindrical mass 

 of small uniform cells with well-marked nuclei covering the 

 gonidial vessel, but it is impossible yet to say whether they 

 will give rise to ova or spermatozoa. In the adult the gonad 

 is a club-shaped or cylindrical structure, 1'3 mm. to 1'5 mm. 

 long, surrounding the gonidial vessel (PL 26, figs. 53, 54). 

 The first nephridium has a well-developed gonidial vessel, 

 but no genital cells are formed upon it. The cells covering 

 the vessel are brown in colour, due to the presence in them 

 of chlorogogenous granules. 



The gonads of A. ecaudata differ considerably from those 

 of any other specieSj being larger, more complicated, and 

 more numerous, there being twelve pairs (PL 26, fig. 45). 

 On each side of the nephrostomes of this species there is a 

 strand of cells which, owing to the numerous well-marked 



