750 NATURAL HISTORY J¥ AQUATIC ANIMALS. 



as to gire rise to the elevations spoken of. Besides this, the inner table had become so weakened 

 at the insertion of the adductor muscles that the animal in closing had torn a part of it loose, 

 which had been repaired by the deposition of a brown, horny substance. Evidence of the presence 

 of the boring sponge may very frequently be noticed in shells of Oysters brought to the markets, 

 though it often appears as if the parasite had left its work incomplete, being killed on its host. 

 I find that Schmidt has also noted this, and that the boring operations of the sponge usually seem 

 to stop in the case of living mollusks at the nacreous layer. 



Upon examining some Scotch Oysters, obtained for me for study by Professor Baird, I was 

 struck with the fact that every one was infested with this organism. The effect of the parasitism 

 was that all of the specimens had abnormally thick shells, due evidentlj' to the effort made by 

 the Oyster to deposit more and more calcareous matter in order to exclude its persistent tormentor. 

 Internally the shell showed irregularities due to the intrusion of the sponge. It is highly probable 

 that in this case the growth of the Oysters had been impeded by the parasite, in consequence of 

 the effort made by the animals to exclude their enemy by .increasing the thickness of their shells. 

 This same tendency to increase the thickness of the valves I have noticed in specimens of our 

 native Oyster, the shells of which were infested with this parasite. It is very remarkable that the 

 Oyster should make an effort to exclude its enemy by such a means ; and it is not less remarkable 

 to observe that the lime carbonate secreting function of the mantle is often stimulated to extra, 

 exertion long before the parasite has actually intruded into the cavity of the shell. 



Dr. Leidy gives a lucid account of the living sponge as found in Ostrea virginiana and Venus, 

 mercenaria. He says: "This boring sponge forms an extensive system of galleries between the 

 outer and inner layers of the shells, protrudes through the perforations of the latter tubular 

 processes, from one to two lines long and one-half to three-fourths of a line wide. The tubes are 

 , of two kinds, the most numerous being cylindrical and expanded at the orifice in a corolla form, 

 with their margin thin, translucent, entire, veined with more opaque lines, and with the throat 

 bristling with siliceous spiculse. The second kind of tubes are comparatively few, about as one is 

 to thirty of the other, and are shorter, wider, not expanded at the orifice, and the throat unob- 

 structed with spiculae. Some of the second variety of tubes are constituted of a confluent pair, 

 the throat of which bifurcates at bottom. Both kinds of the tubes are very slightly contractile, 

 and under irritation may gradually assume the appearance of superficial, wartlike eminences 

 within the perforations of the shell occupied by the sponge. Water obtains access to the interior 

 of the latter through the more numerous tubes, and is expelled in quite active currents from the 

 wider tubes." 



The boring process seems to be effected by the action of the living soft material of the sponge, 

 according to observations which have recently been made by a Russian naturalist, according to 

 whom it appears that the calcareous matt^ is dissolved away by the parasite. I am told by a 

 practical oysterman that a bed once planted with Oysters which are badly infested by the boring 

 sponge is apt to remain so for some time, and that the beds adjoining become infested, for the 

 reason that the embryo sponges, which are thrown off in large numbers from the infested "plants," 

 swim about in the water, attach themselves to other Oysters, to begin their injurious growth and 

 excavations in sound shells. 



220. NATURAL AND ARTIFICIAL OYSTER-BANKS. 



Chaeaotbeistics of natueal oystee-banks.— I have examined a number of oyster- 

 banks, which were readily accessible in shallow water, with gratifying results as to the habits 

 of the animal under virtually undisturbed conditions. These banks, like those formed by the 



