342 NEREIS (NEREILEPAS) FUCATA. 



has an altered superior division of the foot (absence of the superior ligula) with simple 

 capillary bristles, and the jaws have disappeared, whilst there is a third median tentacle. 

 This species was found by the late Prof. Griard 1 in the tunnels of Balanoglossus. A 

 survey of other commensals amongst the Polychsets leads to a similar conclusion as regards 

 structural alteration. Moreover, the groups in which this habit has been developed are few, 

 the most conspicuous being the family of the Polynoidas which haunt the tubes of 

 Terebellids and Clrdetopterids, the ambnlacral grooves of Echinoderms, grooves in 

 mollusks, and occur on Hydrocorallines or within the framework of Venns's Flower Basket. 

 The Hesionids have few examples, the best known being Ophiodromus flexuosus on 

 Astropecten and Luidia. Syllids, again, are common in sponges, the unique Syllis ramosa 

 permeating the canals with its branches; Eisig 2 excludes Polydora in the group " Endo- 

 Commensalen " since it perforates oyster-shells so extensively, but it also does the same 

 in various rocks, and in such a case is no more a commensal than Oliona. 



In Montagu's MS. volume of drawings (1808) in the Linnean Society this species is 

 represented in Plate LI, fig. o. 



Chamisso and Eysenharclt 3 (1819) describe from Unalascha a Nereis heteropoda which 

 appears to resemble Nereilepas fucata very closely, though the authors place the change in 

 the form of the foot at the forty-third. They may, however, refer only to the full 

 development of the altered foot. 



In accordance with their views Audouin and Mime Edwards (1834) considered the 

 enlarged dorsal lamella as specially branchial. Their Nereis podophylla is a female 

 epitokous form of the present species. 



The Nereilepas fused of CErsted (1843) is probably the same form. 



Ehlers (1868) did good service in rectifying the synonymy of this species. 



Claparede (1868) adopted Nereilepas as a sub-genus, following Johnston and Malmgren 

 rather than CErsted, Kinberg, and De Quatrefages, though Stratonice, Malmgren, was 

 included. 



He describes two species of the genus from Naples, viz., N. parallelo gramma (the 

 A T . pulsatoria of Grube), and N. caudata of Delle Chiaje. Both species seem to be in 

 need of revision and accurate comparison with N. fucata. Claparede saw vibratile 

 cilia on the lamellas of the latter species leading by grooves to apertures out of which he 

 supposed the sexual elements escaped. This needs confirmation. In Delle Chiaje's 

 figure of what appears to be a foot (Plate Oil, fig. 1 5 4 ) the structure is inverted. 



Wiren (1888) gives a careful account of this form in the atokous and epitokous 

 conditions, indicating the changes in the feet and body-wall respectively. He terms the 

 epitokous form var. inquilina. The diminution of the muscular layers in this condition is 

 marked, as in other annelids under the same circumstances. It occurs, as in Britain, in 

 shells covered with Hydractinia and inhabited by hermit-crabs. 



1 < Compt. Rend./ August 21, 1882, p. 389; and ' Ann. Nat. Hist./ 5th ser., vol. x, p. 330 (as 

 Anoplonereis Herrmanni) . 



2 < Fauna u. Fl. Neap./ xxviii, p. 167. 



3 'De Anim. Quibusd./ etc., Fascic. ii, p. 349, Taf. xxiv, fig. 2, 1821. 



4 'Descriz. Anim. Invert./ vol. iii, p. 96, Tav. cii, figs. 10 and 15, and < Mem./ vol. ii, pp. 403 and 

 426, Tav. xxviii, figs. 100, 115. 



