t 325 ] 



NOTE ON POLYDORA ARM ATA, Lnghs.* 



BY 

 ARNOLD T. WATSON, F.L.S. 



The specimens referred to in this note were found living commensally with a sponge, 

 Aulospongus tubvlatus, which is very common on the pearl banks, and is mentioned 

 by Professor Dendy in his Report upon the Sponges in this series (' Ceylon Pearl 

 I \ ster Report,' Part III., p. 176). 



A fuller description, with a figure showing the tubes piercing the sponge radially 

 and the worms in situ, had previously been given by Dendy in his " Report on 

 Sponges from the Gulf of Manaar" ('Ann. and Mag. Nat, Hist.' (6), hi., p. 73, 1889). 



The general characteristics of the species are well described by Mesnil (' Bull. Sci. 

 France et Belgique,' tome xxix., 1896, p. 203), and I have but few points of difference 

 to note. Of these the most important, perhaps, relates to habitat. 



The specimens described by Mesnil were obtained from Lithothamnion, through 

 which they had pierced and in which they had formed tubes of calcareous sediment ; 

 while Carazzi and Lo Bianco report these worms as living in the shells of Venus 

 and therein forming U-shaped tubes. A similar variation in habitat occurs in another 

 closely allied species of this genus, Polydora cceca, which sometimes lives commensally 

 with the sponge Microciona plumosa, as described by Hornell (' Nature,' vol. 47, 

 1892, p. 78). 



Owing to the difficulty of separating the worms uninjured from the sponge, it has 

 only been possible to secure one or two fairly perfect specimens. The following notes, 

 therefore, may be incomplete. 



The Ceylon worm is apparently a smaller form of the species Polydora arrnata, 

 Langerhans. The length varies from 2 to 3 millims. and the number of setigerous 

 segments from 22 to 26. The branchiae, of which I find four pairs, commence, as 

 usual, on the 7th setigerous segment. They are comparatively broad and sometimes, 

 but not always, of equal length. In one instance they gradually lengthen, the first 

 being only half the length of the fourth. Eye-spots may or may not be present ; 

 most frequently they are invisible, but, after special treatment, in one case, I 

 detected one pair. The tentacles are fairly long, reaching, in one specimen (though 

 bent and twisted), to the 7th setigerous segment, 



The setae of the 5th segment, each with a characteristic hook, arising from an 

 upright collar, which terminates on either side in a prominent pointed process 

 projecting in the same direction as the hook (the whole having somewhat the 

 appearance of an articulated seta), correspond well with Mesnil's figure. They 

 number two, or sometimes three, in each parapodium. 



* Langerhans, ' Zeitschr. f. Wiss. Zool.,' 34, 1880, p. 93. 



