REPRODUCTION IN THE UNIONID.^i] 
95 
borne out by the fact that the ''axe-head "can be readily imagined 
as a modification of the glochidial outUne seen in some species of 
Lampsilis, which like subrostratus show an approach to a rectan- 
gular outline. Its four hooks are so arranged that those of one 
valve pass inside the opposite ones, thus bringing the ventral mar- 
gins close together and giving a very firm hold upon the host's 
tissue. In other respects, it does not show marked differences 
from the hookless type and the few experiments we have made 
with it indicate its attachment to the gills rather than the fins. 
The detailed structure of each type of glochidium, as found in 
Anodonta and Unio, respectively, has been quite well known for 
many years and recently Harms ('09) has studied them with even 
more care. We have nothing to add to this except that we have 
observed concerning the larval thread (formerly erroneously 
termed the byssus), which we will mention, since the current 
accounts in zoological text-books and literature lead one to believe 
that this structure is a conspicuous feature of all glochidia, which 
is not the case, although it does happen to be present in the 
European forms which have been most studied. We find the 
larval thread present in the species of Unio and Anodonta which 
we have been able to examine with care and the thread is probably 
a characteristic of these genera. We have never seen any sign 
of such a structure in the ripe glochidia of the other genera above 
Usted as possessing hookless glochidia, nor in the hooked forms of 
the genus Symphynota. Lillie ('95, p. 52) considers the thread a 
condensed excretory product, which, accepting the account of 
Schierholz ('88), he thinks has become an organ which is of use 
in bringing the glochidium in contact with the fish. This latter 
function is the one commonly ascribed to the thread. While 
we have not studied the pre-glochidial stages in the development 
of those species which show no thread gland in the mature glo- 
chidium (although it is important that this should be done with 
a view to determining whether any homologue of the thread 
gland is present at any time), we do not know, that repeated exam- 
ination of glochidia, either ripe or well along in their develop- 
ment, in several species of Lampsilis, particularly ligamentinus, 
rectus, anodontoides, ventricosus, luteolus and subrostratus, and 
