HORNELL— ANATOMY OF PLACUNA 9l 



dark brownish glands upon the auricles of the pearl oyster. If my theory be correct 

 then we have in Placuna, in these accessory excretory glands, the homologues of Sassi's 

 ciliated funnels and the sole vestiges of the ccelomic cavity. 



Were we to suppose, as is not unreasonable, that originally Placuna had a fairly 

 normal pericardial ccelomic space, with a group of accessory (pericardial) glands on the 

 inner surface of the pericardial wall towards the reno-pericardial apertures, then we have 

 now a condition where the pericardial cavity has become obliterated with a concurrent 

 development of the pericardial glands on each side of the body around what was 

 originally the reno-pericardial aperture. The present extreme forward disposition of 

 these glands, and conseq^uent remoteness from the ventricle, is of no force against this 

 view for two reasons, lip-mely — (a) the auricles, which it is difficult to differentiate 

 from the common efferent branchial trunks, reach as far forward as the glands in 

 question — indeed the base of each gland is connected with the upper wall of the efferent 

 branchial trunk, and (b) in several molluscs, as in Margaritifera, each reno-pericardial 

 canal is so elongated forwards that its aperture is close to the anterior apex of the. gills, 

 a position similar to that of the glands present in Placuna. The aperture of each of 

 these accessory excretory glands may be considered as the morphological equivalent of 

 a reno-pericardial aperture of more normal types, and the glands themselves as the 

 sole remnant of the ccelomic cavity. 



Yet another extreme peculiarity seen in Placuna is the presence of a large 

 quantity of delicate tissue intermingled with the tubules and ducts of the gonad. It 

 appears to be too abundant to be regarded as constituting a stromal framework, but so 

 far I am unable to determine definitely its significance. The one suggestion I have to 

 make is that it possibly constitutes a food reserve, to be drawn upon when the condition 

 of the water in which the individual lives is unfit for some reason to supply a 

 sufficiency of food. For example, I believe that with many molluscs there are periods 

 when, on account of excessive turbidity due to the suspension of great quantities of 

 mud particles, feeding becomes so difficult and unsatisfying that this function is 

 temporarily suspended, either wholly or in part. In the case of pearl oysters (M. 

 vulgaris) in aquaria I have noticed a cessation of feeding to occur whenever on 

 occasions the water supply (which passed to the aquaria without filtration) became 

 charged with an excessive amount of fine sediment. In such cases, although the 

 particles would be collected upon the gills and conveyed to the palps, these organs, 

 after carefully forming them into pellets, invariably rejected them as unfit, presumably, 

 for food purposes. Placuna lives under conditions where every flood carries quantities 

 of fine sediment into its favoured backwaters, and where, too, tidal influences and 

 wind-lop are often sufiiciently strong to disturb the light sediment resting on the 

 bottom of the shallows. Under such circumstances it would appear to be of distinct 

 advantage to an animal to have a food reserve which would permit of temporary 

 cessation from feeding when the condition of the medium in which it lives 



