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POPULAR SCIENCE REVIEW. 
of which contains a dark nucleus giving the reaction of uric 
acid, the salts of which not unfrequently form concretions, or 
calculi, in the organ. M. Lacaze-Duthiers, however, though 
not denying that part of the function of this organ may be 
renal, considered that it has also some intimate connection with 
the state of the generative gland ; and has, accordingly, termed 
it “ annexe de la generation.” 
The breathing organs consist of a pair of light brown gills, 
or “branchiae,” on either side (fig. 1 , r g, l g ), which may be 
regarded as modified inner folds of the mantle (fig. 6 , o < 7 , i < 7 , 
m). Each gill further consists of two leaves, or laminae, 
separate at the attached extremity of the organ, but adherent 
along its free edge, thus enclosing a kind of sac, across which 
transverse bars run at intervals from one wall to the other. The 
innermost laminae of the right and left inner gill, instead of 
being confined, as are the rest of the laminae on either side, to 
a line of attachment just outside of the organ of Bojanus, meet 
together in the middle line posteriorly to the foot, and so form 
a kind of partition which fences off the branchial from the 
anal chamber. As each gill cavity is divided into a series of 
longitudinal canals by the transverse septa, so is a system of 
transverse pores formed by the intervals between the approxi- 
mation of neighbouring nodulous thickenings of certain bars 
which may be seen, even with the naked eye, running from 
before backwards along the gills. Kunning at definite intervals 
across the outer side of a gill-lamina are certain lines which 
indicate the course of vessels returning the purified blood 
heart-wards, while within the gill-sac are visible other trans- 
verse lines of vessels, running in the intervals between the 
lines first described ; these bring impure blood to the gills from 
the so-called “ renai-portal ” venous plexus of the organ of 
Bojanus. These efferent and afferent trunks form by inoscula- 
tion a mesh-work bounding the pores and channels of the 
gill-sac, which are, as well as the free edge of the organ, richly 
fringed with cilia. 
Generative System . — The sexes, as Leuwenhoeck was the first 
to discover, are distinct in the river-mussel and the rest of the 
Laraellibranchs. The generative glands, which are packed away 
in the foot among the other viscera, occupy, as before stated, 
blind sacs opening laterally into the twigs of the water-vascular 
tree. There is no sexual congress ; but the male secretion, a 
milky fluid full of spermatozoa, after being discharged into the 
surrounding water, is drawn in by chance with the current 
which enters the branchial chamber of the female, and so 
eventually reaches the ova. In the summer the ova, which 
are globular and transparent, and have a firm shell, are ex- 
truded from the generative orifice (fig. 4, 0 v ), and conse- 
