1882.J 



MICROSCOPICAL JOURNAL. 



83 



overlooked, and, with a view of ob- 

 taining such evidence, but without 

 the slightest prejudice as to which 

 way it might lead, I procured some 

 oysters that had recently been taken 

 from the beds of the East River — a 

 considerable number having shells 

 tenanted by Cliona. 



An exhaustive microscopical ex- 

 ination of these and similar speci- 

 mens, which has occupied my leisure 

 time for several months, presents the 

 following features, which seem to me 

 to establish, beyond a possibility of 

 doubt, that the sponge is, in this case 

 at least, the only factor to be held ac- 

 countable for the burrows. The 

 outer layer of these shells was punc- 

 tured with numerous holes, often many 

 hundred, varying from the -^ to the 

 Y^^ of an inch in diameter, generally 

 occupied by the osculae of the sponge. 

 Between the outer and inner layers, 

 and extending laterally, the shell was 

 almost entirely excavated, and the 

 space occupied by the sponge and its 

 numerous spicules ; while extending 

 inward from this sponge-mass were 

 innumerable minute, branching and 

 ramifying burrows, uniformly and 

 completely filled with corresponding 

 arms of sponge, many of which ex- 

 tend quite through the interior layer 

 of shell, and are plainly to be seen 

 under the microscope. The contact 

 of these arms of the sponge with the 

 external membrane of the oyster causes 

 the latter to deposit at such points an 

 additional amount of lime carbonate, 

 and the interior surface of such shells 

 presents the appearance of numerous 

 little prominences caused thereby. 

 The oyster in some cases seems to 

 succeed in barring out the intruder, 

 but in several of the shells here 

 shown, the sponge, in almost every 

 case, passes quite through. In view 

 of the evidence presented by these 

 shells, of the quantity of lime carbon- 

 ate heaped about the puncture 

 through the inner layer, it is apparent 

 that the withdrawal of the animal ex- 

 cavating them would be followed, al- 

 most immediately, by the closing of 

 such apertures. 



The only possible theory that will 

 account for these burrows, if they 

 are not made by the sponge, is that 

 they are the deserted excavations of 

 annelids, or other marine worms ; and 

 from the conditions here shown this 

 theory is utterly untenable, for we 

 shall be required to believe that this 

 shell was once inhabited by an in- 

 numerable multitude of such worms ; 

 otherwise the perforations through the 

 inner shell would have been closed, 

 and all of these must have retreated, 

 at the same time, so completely that 

 no trace of them could be found, and 

 the sponge must then have extended 

 its growth into the deserted channels 

 with such rapidity as to fill every 

 minute branch before the oyster 

 could bar it out by the secretion of 

 enough new shell to stop apertures 

 the TffW of an inch in diameter. 



But this is not all. The burrows 

 occupied by Cliona branch in all di- 

 rections and diminish in diameter as 

 they extend inward, which would re- 

 present a method of boring quite in- 

 consistent with the habits, or even 

 the possibilities, of annelide, Scolytus, 

 or any other known borer. Again, in 

 these specimens, the sponge was 

 found in small spots on the thin la- 

 minae, around the sides and anterior 

 edges of the shell which represent its 

 most recent external growth, and in 

 such cases the laminae were perfor- 

 ated from side to side. 



Respecting the concave depres- 

 sions bounding excavated parts, I 

 can only say that the large annelids, 

 which, so far as I have been able to 

 discover, are the only occupants of 

 these shells except the sponge, are, 

 from the character of their jaws, 

 incapable of cutting such depres- 

 sions, and, moreover, from their 

 size, unable even to enter the 

 burrows. These annelids have a cu- 

 rious habit of boring into the shell a 

 certain distance and then turning or 

 doubling upon their track ; some- 

 times sweeping around a circle, they 

 come out through a course so near that 

 by which they entered as to cut the 



