THE ANATOMY OF THE HI YER-MU SSEL. 
249 
the blood flows in channels hollowed out in various parts of the 
body. The researches, on the other hand, of Prof. Rolleston 
and Mr. Robertson, which appear to be very reliable, in that 
they consist mainly of careful injections, tend to prove that 
the blood-vascular system is a closed one. “ The blood-vessels,” 
they say, “ seem to us to constitute* a system of tubes closed, 
save at one point and at one lacuna. That point and that 
lacuna is the pericardial space — a cavity into which, besides 
the blood of the animal, the water in which it lives also finds 
its way.” The normal course of the blood is therefore as 
follows : it flows through the aorta into a true capillary system, 
thence into the systemic veins which terminate at a sinus 
lodged within the foot, which in turn communicates by an 
orifice with a second sinus already described as lying between 
the two lateral factors of the organ of Bojanus ; it then tra- 
verses “what may be called the renal-portal system of the 
organ of Bojanus ;” and, lastly, reaches the gills, there to be 
aerated, and thence to return, laden with oxygen, to the heart, 
by way of the branchial veins and auricle of either side. 
The external orifice of the pleural sac of either side, naturally 
patent in Unio margaritifer , cannot be well seen in Anodonta 
without separating the two laminae of the inner gill, when it 
will be found situated not far behind their anterior extremity. 
Just on the inner side of this orifice is another opening (fig. 
4, o v\ leading into a canal, the direction of which is such, 
that a bristle passed into it will lie at right angles to one in- 
troduced into the pleural sac. This canal is continued into a 
system of tubes which ramify in the substance of the foot, and 
to which blind sacs, or caeca, lodging the generative glandular 
elements, are appended laterally. This is the aquiferous or 
“ water- vascular” system; and its orifice just described acts as an 
outlet, as well for the product of the generative glands as for the 
fluids which course through its channels. This system is princi- 
pally concerned in the great distension, preliminary to loco- 
motion, of which the foot of the animal is capable, and which 
appears to be effected in this wise. When the shell gapes, 
the pericardial sac is necessarily dilated, and into the partial 
vacuum thus caused water from without passes, after first 
traversing the pleural and glandular sacs of the organ of 
Bojanus, and then makes its way into the systemic veins, 
whence it appears “ to transude into the system of water-tubes 
everywhere in apposition with them.” 
The “organ of Bojanus” was considered by its discoverer to 
be the sole respiratory organ, the gills being regarded by him 
as only receptacles for the ova, whence he termed them “ Brut- 
halter.” It is now generally held to be a renal organ, or kidney, 
since the folds of its glandular sac are lined with cells, each 
