306 Prof. M'luto.sh'd Notes from the 



instance, gives no evidence that by the " constant move- 

 ments in and out " the slightest change lias occurred in their 

 diameter. Nor would the same movements alter the diameter 

 or make the tunnels of Polydora or Dodecaceria. The power of 

 boring is a s|iecial attribute of these and other forms which 

 ])erforate rocks and shells. It might be as appropriately said 

 of Teredo -mmX Xijlophaga (mollusks which perlbiate wood) that 

 their tunnels are maile by " constant movements in and out." 



Polydora is an active borer in shells, rocks, and stones, ami 

 if it be carefully observed when working in the softer pieces 

 of shale, cloudy jets of the dtbiis are noticed issuing from the 

 apertures of its tubes at intervals, and they form a deposit on 

 the bottom of the vessel. Whether this consists wholly of 

 materials from the excavations in the shale or partly ot the 

 contents of the intestine is an open question, but it partakes 

 of the debris of the shale. Dr. Carazzi's supposition that 

 the description given by the writer* might apply to chinks 

 and fissures between the layers of the albuminous shale 

 is unnecessary. The remarks applied to solid and fairly hard 

 })ortions of the shale, which required to be split to shnv the 

 tubes of the annelid, just as in th^ hard calcareous rocks. 



Oysters (and other shelly mollusks) are everywhere attacked 

 by Folydonij but such can scarcely be termed a disease. 

 The annelid is a commensal in the case of the oyster, and 

 shares with it the muddy food carried by currents within 

 reach of its mouth and tentacles, which are richly ciliated and 

 fitted for conveying" particles of various kinds. It settles on 

 the oyster because it possesses a shell wherein it can bore, 

 and because the shell tish is usually on a site suitable for 

 supplies of Ibod. Some, such as Polydora ciliata, do not neces- 

 sarily interfere with the inner layer of the valves, and thus 

 are in contrast with the boring sponge [Ciiona], which not 

 only is more destructive to tlie mass of the shell, but per- 

 forates the nacreous (innerj layer by multitudes of minute 

 pores, necessitating a constant secretion from the outer layer 

 of the mantle to close them, and by-and-bye rendering 

 the ir.terior of the valves discoloured, nodulated, and un- 

 sightly. At first each minute pore is shut off by a thin 

 circular patch of the secretion, but as the sponge pertinaciously 

 progresses in its work of destruction the valves assume the 

 condition just mentioned. Ou the other hand, Polydora 

 hoplura^ a southern species, as shown by Dr. Carazzi, more 

 frequently perforates the valves and causes collections of 

 mud and the formation of blisters as described by Mr. White- 

 legge. As I\Ir. Whitelegge states, the general view is that 

 the presence of Polydora on oyster-beds is connected with 

 * Ann. & Mag. Nat. IIi.<t. ser. 4, L"?bS, ii. p. »'70. 



