POTTS— POLYCH^TA. 311 



thirty-third, and it is interesting to note that in the case of Foli/noe pyknolepis, Gr., 

 quoted hy Duncker *, elytra are found on both these segments, though otherwise the 

 distribution is different. The chief difference between H. zeylanica and the species 

 here described lies in the detailed structure of the ventral setae ; in the former they are 

 smooth, in the latter with slightly spinose apex. 



Genus IPHIONE. 



20. Iphione murioata (Savigny). 



Polyrwe muricata, Savigny, Syst. des Annelides, p. 21. 



Pohjno'e peronea, Sclimarda, 1861, Neue Tvirbellose Thiere, p. 157 [Ceylon]. 



Iphione muricata, Kinberg, Of vers, af Vet.-Ak. Forhand. 1855, p. 383 ; Grube, Annulata Semperian;!, 



p. 21 [Philippines] ; Gravier, Ann. Mus. Hist. Nat. t. iii. 1901, p. 226 [Red Sea] ; Willey, 



in Heidman, Ceylon Pearl Oyster Report, pt. iv. 1905, p. 246. 



This species has been recently very fully described by Gravier and by Willey. It 

 evidently has a wide distribution in the Indian Ocean, north to the Red Sea, west to 

 the Philippines, and east to Zanzibar. In the present collection it is represented by a 

 large number of examples from Minikoi and the Maldives, a few from the Seychelles, 

 and several from Zanzibar. It is one of the commonest Polychsets of coral-reef's. 

 Mr. Gardiner has furnished me with some interesting details regarding the biology of 

 the species. The worm is generally found amongst coral-sand, but preferably in a 

 sheltered situation, such as would be afforded by rocks and boulders resting on sand, 

 though it may sometimes be seen sticking on the rocks. During the night it leaves 

 its diurnal station and wanders on the reef, returning, however, as proved by actual 

 observation, to the same position at daylight. It thus provides an excellent example of 

 a " homing " instinct. 



Localities. Minikoi. Maldives. Seychelles. Zanzibar. 



One of the specimens from Addu Atoll, Maldives, was picked off a " white-brown 

 Holothurian " in company with a Hesionid and another Polynoid. 



Genus GASTROLEPIVIA. 



21. Gastrolepidia clavigera, Schmarda. 



Gastrolepidia clavigera, Schmarda, Neue wirbellose Thiere, 1861, p. 159. 

 Gastrolepidia amblyphyllus, Grube, Annulata Semperiana, 1878, p. 46 [Philippines] . 

 Gastrolepidia clavigera, Willey, in Herdman, Ceylon Pearl Oyster Report, pt. iv. 1905, p. 253. 



No account has been given, as far as I can find, of the association of this animal with 

 holothurians as a commensal. In its most abundant form in this collection the worm 

 is a deep purple-black ; but the dark coloration is relieved by the white ends of the 

 clavate dorsal cirri and white spots upon the elytra. It thtis exactly matches the sea- 

 cucumber Holothuria atra *, on the surface of which it lives. Here there also occurs 

 a similarly coloured Portunid crab, Lissocarcinus orbicidaris, Dana. Another form of 

 the worm is characterised by its much lighter coloration, the elytra being white with a 

 few brown specks. This was found at Naifaro, Fadifolu, associated with two kinds of 



* This common coral-reef form has a thin test which it covers with sand. 

 SECOND SERIES. — ZOOLOGY, VOL. XIII. 4-5 



