REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 15I 



bookless type, although the outlines of the two glochidia are very similar when seen on 

 edge, as in their figures la and 2a of plate i. With respect to the significance of these 

 facts when applied "to a relationship between Icevissima and capax," they conclude that 

 "there would be strong corroborative evidence in adult characters alone" for the closer 

 union of these three species, and this "in spite of the fact that lavissuna and capax are 

 the two extremes in the degree of inflation." The similar degree of inflation of capax 

 and ventricosa offers, they believe, "only a striking instance of convergence in one 

 character." 



THE LARVAL THREAD. 



Our observations upon the occurrence of the larval thread (formerly erroneously 

 termed the byssus) are of importance, since the current accounts in textbooks and 

 literature lead one to believe that this structure is a conspicuous feature of all glochidia. 

 Such an assumption is natural because the organ is conspicuous in the European ano- 

 dontas and unios and in the American species of these genera examined by Lillie (1895). 



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 undoubtedly a characteristic of 

 these genera. We have never seen any sign of such a structure in the ripe glochidia of 

 the other genera, above listed, which possess hookless glochidia, nor in the hooked forms 

 of the genus Symphynota. Lillie (1895, p. 52) considers the thread a condensed excre- 

 tory product, which, accepting the account of Schierholz (1888), he thinks has also 

 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. We have not studied 

 the pre-glochidial stages in the development of those species which show no thread- 

 gland in the mature glochidium, although it is important that this should be done with 

 a view to determining whether a homologue of the thread gland is present at any time. 

 We have, however, made repeated examinations of glochidia, either ripe or well along 

 in their development, in several species of Lampsilis, particularly in ligameniina, recta, 

 anodontoides , ventricosa, luteola, and subrosirata, and to a lesser extent in species of the 

 other genera mentioned, without finding any trace of the thread which is so conspicuous 

 a feature of the glochidium of Lhiio complanatiis . 



We have also examined the glochidia of Symphynota complanata many times with 

 the same negative results, and a smaller number of observations confirm this for S. cos- 

 lata. Since many species thus have no thread in any way functional for attachment to 

 the fish, the question arises whether the thread when present has as important a func- 

 tion in this respect as has been supposed. Our observations upon the glochidia of 

 Anodonta cataracta confirm the descriptions of Schierholz (1888) and others who have 

 studied the European species of Anodonta as to the tangling of the glochidia into masses' 

 by means of their extruded threads, and in this genus the threads do seem effective in 

 drawing other glochidia into contact with the fish when a single one has become attached. 

 This is not, however, effective during the greater part of the period in which the glo- 

 chidium may remain alive upon the bottom, for the threads are dissolved within a day or 



