282 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
The sexual system consists of racemose glands within the abdominal sac, the 
male and female organs being so much alike that only a very clever expert can tell 
them apart. The sexes are generally, though not always, separate. Usually one ani- 
mal is a male and another a female, and this is always the case among the more highly 
organized ISfaiades, but it has been pretty well proved that in some cases individuals 
are provided with both sets of organs. It is held by some students that certain of 
these mussels may change from one sex to the other, but this has not been proved. 
It is not certainly known yet how impregnation takes place, but it is supposed that 
the male ejects the spermatozoa into the water, where it is taken ni) by the female 
and passed into the ovaries. Within the ovaries the eggs are developed, and when 
they reach a certain stage they pass down through an opening into the gills. Here 
they undergo still further changes, developing a small bivalve shell, but very different 
in form from what it is when mature. Later on they are thrown out of the ovisacs 
of the transformed gill and fall to the bottom of the stream or pond. 
Each little mussel is usually provided with one or more pairs of spines or hooks on 
its base, and when thrown out it lays on its back with the valves opened very wide. 
A long filament 
floats up from the 
minute clam, which 
in some way can at- 
tach itself to the fins, 
gills, or scales of 
fishes which come 
in contact with it. 
When such a con- 
nection is made, the 
little mussel rapidly 
draws itself up and 
snaps the valves on 
Fi(i. 4. — Vnio gibbosus Barnes. A species in which the male and female shells are alike. the gill SCale Or 
Tliis has Young filling the outer gills only. , ” I ’ 
tin, burying the 
hooks in it. This irritates the fish, causing it to throw out a fleshy substance, which 
incloses the embryo naiad just as an oak leaf covers u]> the egg of the gall insect 
that is laid in it. This cyst is air and water tight, and in it the little prisoner remains 
for a period of some seventy days, growing but little, though developing its organs. 
It is easy to see that during the time of its eucystment it may be carried many miles 
away from the spot in which it left its mother’s gills, and when it finally works out 
and drops to the bottom it is ready to found a new naiad colony. 
It has often been a soui’ce of wonder to naturalists how a given species of fresh- 
water mussel can inhabit a number of independent streams flowing into the sea, for 
it is a well-known fact that the JSfaiades can not live in salt waiter or even that which 
is more than very slightly brackish. Yet such is often the case. Unio complanatus 
inhabits every stream emptying into the Atlantic from Labrador to Savannah. And 
Anodonta caUformensis is met with in a great number of rivers flowing into the Pacific. 
Now, I conceive that it would be easily possible for these embryos to attach them- 
selves to the marine fishes which go up the rivers that empty into the sea and into the 
fresh water to spawn; that in some cases — rarely perhaps — these fish, not finding 
conditions favorable for spawning in the first stream entered, might return to the sea 
