302 Prof. M'Intosli's Notes from the 



considerable irritation ensues when, tor instance, a Poh/dora 

 makes use of a fissure at the margin of the valve, follows the 

 chink inward, and may even penetrate to the mantle, or when 

 in tlie course of its boring it perforates the valve. The mud 

 ejected by the vent may thus collect and require investment 

 bv the shelly secretion, and cause the so-called blister. A 

 large blister H inch long in a specimen sent by Mr. Want 

 may thus have originated, and it had a double shelf, indi- 

 cating that, alter the first source of irritation had been shut 

 off, a new intrusion had occurred. The tubes wiiich led to 

 this were only sunk in the shell at the extreme margin 

 and for a short distance, the rest of the communication with 

 the interior being between the laminae, where a chink existed, 

 probably the result of the former irritation. Such a condition 

 is very apt to happen now and then under the varied circum- 

 stances affecting tliousands of annelids and oysters, aiid had 

 been described by Pi of. Haswell and seen in an example 

 from the same Australian grounds (Newcastle) in 1883. 

 But it must be borne in mind that blisters may arise in 

 shells where no examples of Polydura have been or are. 

 Thus in a very fine oyster from Lake Cutgee, near Bermagui, 

 Australia, a large blister was caused by a group of small 

 pebbles, the largest nearly reaching a quarter of an inch and 

 the smallest like a coarse sand-grain. In certain very fine 

 oysters from Whitstable thin blisters occur without apparent 

 cause, and no Polydorce are present. It is possible, there- 

 fore, that occasionally Polydora may take advantage of a 

 space caused by other sources of irritation than itself, though 

 there is no reason to suppose that it does not now and then 

 also cause by its excreted mud the irritation, especially 

 Polydora hoplura, the form found by Dr. D. Carazzi in the 

 Australian oysters. In some examples of this kind it is 

 possible that the annelid found it more convenient to utilize 

 the vacuity in the oyster than send the mud out of the ex- 

 ternal apertures of its tubes. In the great majority of British 

 shells few collections of mud enclosed by a shelly layer are 

 found, and yet the valves may be riddled by Polydora ciliata. 

 The statement in the Report that each worm necessarily has 

 its own collection of mud therefore requires the qualification 

 of species. The CMllections of mud with the young worms 

 within the valves have not been observed in this country, 

 where P. hoplura has not yet been specially noticed. The 

 author discusses the probabilities of the cavities of the blisters 

 being enlarged by the worm, but this has never been supposed 

 by any zoologist. In most instaiices the tuiuiels bored by 

 J'olydira ciliata in shells are cylindrical and quite clean 



