314 



ROBERT TRACY JACKSON ON THE 



fore in favor of the assumption that the cement by which the oyster is attached is wholly 



organic. 



With the introduction of the spat stage, a fundamental histological difference arises 

 abruptly in the structure of the shell of Ostrea virginiana as in O. edulis, fig. 27. As 

 stated abo\a», the prodissoconch consists of lime infiltrating an amorphous matrix of con- 

 chyolin, but in the initial spat stage the fii'St layer of shell of the right valve is deposited in a 

 "tessellated or prismatic" manner in a horny matrix, as desci-ibed and figui-ed by Professor 

 Ryder (62, 64) . Soon after the formation of the spat stage, the subnacreous, white por- 

 cellanous layer begins to be deposited on the yellowish-brown, prismatic layer, first mak- 

 ing its appearance as ii-regular blotches in the centre of the valves. These two layers con- 

 tinue throughout the rest of the life of the oyster, and togethei-, though in disproportionate 

 degree, build up the massive adult shell. The prismatic layer always remains thin, as may 

 be readily seen by sections, or by treating an adult with acid, when the remaining conchy- 

 olin of the prismatic layer will separate from the overlying subnacreous layer like a dis- 

 sected skin. 



It is a remarkable fact that in 

 young Ostrea virginiana ,^ the 

 prismatic sti'ucture is typical of 

 the upper right valve alone; the 

 lower valve presenting a fine 

 gi'anular structure, as in O. edu- 

 lis, fig. 29. In later life in Ostrea 

 virginiana and some other spe- 

 cies, a very thin layer of pris- 

 matic tissue may be found in the 

 left valve though it is for the 

 most part eroded away, but the 

 same layer is thicker and there- 

 fore better preserved in the right 

 valve. The prisms of the right 

 valve of the adult oyster are not 

 as perpendicular to the surface 

 as in the young, fig. 27. There- 

 fore in horizontal section the outline of the prisms presents oblique rather than hexag- 

 onal faces. A similai- obliquity of the pi-isms may be seen in Solemya mediterranea. 



The nepionic or infant stages of molluscs begin according to the classification of 

 stages adopted with the formation of what Professor Owen called the apex of the conch, 

 or true shell. It is shown that the spat marks the beginning of the dissoconch (true 

 shell) in Ostrea; therefore the spat is considered as the nepionic period.' In my pre- 

 liminary ])aper I said the spat of Ostrea virginiana is divisible into five stages which were 

 described at some length. I also said that they are more or less clearly marked in every 



Fig. 28. 



I'lG. 2i 



Fig. 29. 



Via. 27. Youns Ostrea edulis. The prodissoconch is composed of lio- 

 mogcneous and the early dissoconch of prismatic cellular tissue.' The 

 gills are filamentous. 



Fig. 28. Fragment of border of upper valve, enlarged to sliow the 

 prismatic structure. 



Fig. 29. Fragment of lower valve (after Uorst). 



'The relations of structure of the prodissoconch and dis- 

 soconch are the same in Ostrea viryiiiiaiia, also in Perna, 

 I'ecteu and Anomia, as described in the text. 



''In my preliminary paper this same period was consid- 



ered as the "silphoiogic" period. In a footnote to this 

 p.iper Pi'ofessor Ilyatt has given up the use of the term 

 silphoiogic, and has supplied in place of it the term uepi- 

 onic {vide p. 290) . 



