10 REPORT ON OYSTER CULTURE 



cilia appear at the broader ends of the ovoid groups (fig. d), and 

 motion commences ; still no shell is visible, and at this stage the 

 ova pass gradually from the ovaries into the mantle of the oyster. 



The gills, fin, or mantle consist of four inner and two outer lobes, 

 of which the outer are furnished with cilia, and are permeated in 

 every direction with nerves and secretory ducts, for the exclusion 

 of any enemy or injurious matter, and the secretion of the shell. 



The cilia are in incessant motion whilst the shell is open, caus- 

 ing a constant flow of water over the surfaces of the branchia ; 

 these possess no nerves, but are permeated by vessels in which 

 the blood is aerated by the contact of the water. Between these 

 lobes the ova are retained for some days, the mass gradually 

 passing in tint from white through yellow, yellowish grey, and 

 dark grey, nearly approaching black before extrusion. 



When it has attained this stage, the embryos are almost per- 

 fect, and the mucous matter in which they have previously been 

 embedded, will be found to have been absorbed. 



From one to two millions of young are produced by each preg- 

 nant parent during the spatting season, and their extrusion is 

 accomplished by a series of quick openings and shuttings of the 

 parent shells, thereby causing a sudden outflow of water, which 

 carries the young with it. 



The emission continues until all are expelled, the process ex- 

 tending over a period which does not certainly exceed a week, and 

 when emitted in the mature or black spat state, it is as perfect as 

 when it attains its full growth.* Artificial fecundation, such as is 

 practised with salmonida?, is impossible, from the fact that fecun- 

 dation takes place before the extrusion of the ova from the ovaries* 

 and therefore we must conclude that with oysters the utmost that 

 can be done by so-called artificial breeding is not the procuring 

 of artificial impregnation, but only the shepherding of the im- 

 pregnated ova during infancy. 



The young when in a mature state attach themselves immediately 

 to the first clean hard substance they meet with. It has been 

 ascertained that the spat floating in the water for several days is 

 that which from inability of the parent to contain it or from other 

 causes, has been ejected, or escaped before maturity — such spat 

 forms a large proportion of the whole, and during the time in which 

 the spat is free and is becoming fitted for attachment, it is carried 

 by tides and currents to great distances — is devoured by countless 

 enemies, or is driven upon muddy or otherwise unsuitable ground, 

 and so perishes. Under the most favourable circumstances the 

 proportion that never attaches is very considerable. In a natural 

 state the oyster remains attached, during its lifetime, to whatever 

 object it first adheres to, but after an accidental removal it will again 

 adhere to substances with a hard and tolerably clean surface. 



To save the bulk of the spat when free is the great object 

 of oyster culture. 



It may be here stated that all descriptions of molluscs are 

 apparently free for a longer or shorter term ; of this fact the 

 cockle, mussel, winkle, and balanus tribes afford proof. 



* By feeding the oysters when free with coloured matter such as carmine, 

 in the water, the interior economy is made visible, and the whole will be found 

 to agree exactly with that of the adult oyster. 



