112 NATURAL HISTORY SOCIETY OF DUBLIN. 



siphons exist. The Pearl Mussel is found abundantly in the British Isles, 

 in mountain streams, and is still gathered in dry seasons, for the sake of 

 the pearls which they frequently contain. These are generally found 

 in the fold of the mantle, and near the inferior adductor muscle, and are 

 caused by some irritating substance which has been covered layer over 

 layer by nacre ; those in the free part of the mantle are often nearly 

 spherical, and vary in beauty and colour according to that part of the 

 mantle in which they are found ; their size, too, varies, as the source of 

 irritation and the age of the Mussel. A very fine pearl from a "Ohio, said 

 to have been found in Cornwall, is now in the British crown. 



I am indebted to Dr. Carte for the specimens now exhibited ; they 

 were taken in Kerry, and have been submitted to careful examination. 

 Two pearls were found, but they were of no commercial value. It is 

 astonishing how little is known of the anatomy of Molluscs ; and very 

 often (as in this case), when the investigator wishes to ascertain what is 

 known on the subject, he is compelled to refer to obscure periodicals, or 

 to go back to the older anatomists, as Bojanus and Poli, and others of 

 that class, to arrive at original observations. The manuals at present in 

 use are mere compilations, in which mistakes and incorrect diagrams 

 are too often sedulously copied without investigation, while the observa- 

 tions of the older writers are obscured by the terms and diction then 

 in use. 



Those portions of the anatomy of the Unio to which my attention 

 was more particularly directed were the alimentary canal and nervous 

 system ; and as I have not found these important parts either described 

 or figured exactly as they appeared to me, I thought it might be useful 

 to lay these dissections before the Natural History Society. 



The alimentary canal of the Unio is of extreme delicacy, and is for 

 the most part covered by a tough muscular envelope, forming the base 

 of the foot of the animal, which latter consists of layers of muscle spread- 

 ing in different directions, and connected together with distinct fibres, 

 not generally muscular. These distinct fibres in the neighbourhood of the 

 intestine and nervous system form a series of bars transverse to the 

 course of the intestine, and serve, in my opinion, to prevent undue pres- 

 sure on the intestinal canal and nervous system on the contraction of the 

 strong muscular foot; they form also interstices in which the water- vas- 

 cular system is situated, on which depends so much of the turgescence 

 and power of the foot. The presence of these tough trabicular septa, and 

 the great delicacy of the intestinal canal in close approximation to them, 

 render the dissection of this part a labour of difficulty and patience, and 

 require that the investigation should be carried on under water, with 

 the aid of a good lens. The mouth, which is large, admitting a glass 

 rod of about a third of an inch in diameter, is furnished with four leaf- 

 like labial tentacles, and leads to a stomach capable of containing a bean; 

 the walls of the stomach are thin, and covered with a salivary glandular 

 substance, the ducts of which receive freely injection thrown into the 

 stomach; the intestine commences narrow and valvular, and courses 

 three-quarters round the bulk of the liver, making three sharp turns on 



