Gatty Marine Lahoratory ^ St. Andrews, 301 



Z^yf/ecac^r/a, another boring annelid, the single tube of which 

 forms a keyiiole^outline in transverse section. 



The expression that the inlet and outlet of the tube is 

 *' often enclosed by the thin layer of shell deposited by the 

 oyster " proljably means that, according to the author, the 

 uoim stoically keeps its body in the line of its tube whilst 

 the oyster makes a ca.st of it, a conilition of things foreign to 

 experience. It may happen that at the growing edge of the 

 valve the projecting tubes of mud may be fixed by t!»e shelly 

 secretion of the edge of the mantle, but there is no proof 

 that the annelid would thereby be prevented from gaining 

 the exterior — so necessary for its existence. 



'I'he author makes much of the mud in which he appears 

 invariably to have found the young Pohjdora^ and of " the 

 thin membranous covering thereof secreted by the oyster." 

 He has seen no groove indicative of boring in such cases, and 

 denies, indeed, that the annelid bores at all. Such views, 

 however, cannot be received by zoologists either in regard 

 to Polydora or other boring annelids, such as Dodecaceria 

 and Sabella saxicai-a, or in regard to the perforations made 

 by Fhoronis, boring mollusks, and sponges. Polydora bores 

 not only in oysters, but in dense shells, such as Fusus 

 and Buccinum, and over vast areas of the ocean, and 

 is one of the important agents in disintegrating the shells on 

 the sea-bottom. It is true that very little might be visible 

 at first, as the postlarval worm settles on the oyster, 

 but even less would be seen in the case of the postlarval 

 sponge, as it fixes itself to the shell, yet in due time the 

 shell is tunnelled by the one and honeycombed — often 

 with beautiful dendritic patterns — by the other. The idea 

 that the postlarval worm settles inside the valves of the 

 oyster could only apply to the tree edge (uncovered by the 

 mantle), and it is traversed by the fact that there is no such 

 secretion to account for the phenomena in the case of a lime- 

 stone rock or a mass of shale. If the annelid does not bore, 

 how does it make the finely formed tunnels in such rocks 

 and in the thick coating of Melobesia (a calcareous alga) 

 which covers many rocks and stones between tide-marks, and 

 in Pa/(trti? Few traces of mud and no "thin membranous 

 coverings " occur in these instances. 



The view that the jx)stlarval worm, with a great capacity 

 for attachment, simply clings by the mouth to the margin of 

 the shell of the open oyster, " constructs a tube, and collects 

 a large quantity of mud," which the oyster surrounds with a 

 shelly secretion, and repeats it when the worm continues its 

 tube, will not luUy explain the case. There is no doubt that 

 Ann. d: ^fay. X. Hist. Scr. 7. Vol. ix. 21 



