140 



transmitted light, with a linear power of 500. Under these circum- 

 stances the canals may often be seen emerging from between the sides 

 of the cells, sometimes presenting an abrupt coecoid termination, oc- 

 casionally ending in a short bifurcation, or pursuing their courses 

 among them, and occasionally assuming an approach towards a reti- 

 culated form. This resemblance to a truly reticulated structure may 

 be distinguished from the interstitial vessels by the greater size of the 

 tissue, and the incompleteness of the reticulations, which sometimes 

 pass but a third or a half round a cell or cells, while at others the cir- 

 cuit is completed, and the branch either anastomoses with the first por- 

 tion of itself, or abruptly passes downwards between the cells in such a 

 position, as to render its termination or course exceedingly doubtful. 

 An interstitial vascular tissue exists in Ostrea, as well as in Pinna, 

 but it is much more difficult of observation in the former than in the 

 latter. In a small portion of the inner surface of the shell from near 

 its junction with the thin laminated edge, I succeeded, with a power 

 of 1000 linear, in seeing two distinct layers, the one beneath the other, 

 but I could not discern any short connecting vessels at right angles 

 to their planes, such as are so readily to be seen in the analogous 

 vascular tissues of the Pinnce. These vessels are larger but not so 

 symmetrical in Ostrea edulis as in Pinna, and they are frequently 

 distended at intervals by an accumulation of gelatinous-looking mat- 

 ter, as seen in figure 5, PL xiv. which represents a portion of the 

 interstitial vascular system of the shell of the common edible oyster, 

 separated by means of weak hydrochloric acid. The vessels which I 

 have hitherto described are not the only ones which exist in the shells 

 of molluscous and conchiferous animals, for there is scarcely a mem- 

 branous film that can be separated from the animal remains obtained 

 by the maceration of these bodies in a weak solution of hydrochloric 

 acid, in which exceedingly fine ramifying vessels may not be observed, 

 provided a sufficiently high power be used in their examination; but 

 this is absolutely necessary to a successful investigation of their 

 structures, which requires powers varying from 500 to 1000 linear to 

 ensure distinct and satisfactory results. After the description of the 

 tissues already named, it is scarcely necessary to say, that, as a mat- 

 ter of necessity, there must be a free vascular communication be- 

 tween the animals inhabiting both univalve and bivalve shells and 

 their habitations, through the medium of their points of attachment : 

 but this fact is exceedingly difficult of demonstration ; and, although 

 1 have used my best endeavours to trace the vessels both from the 

 animal to the shell, and from the shell to the animal, I have not yet 



