146 



BULLETIN OF THE BUREAU OF FISHERIES. 



THE HOOKLESS TYPE. 



Since the greater part of our experimental infections with glochidia of the bookless 

 type have been made with our common species of Lampsilis, we have examined the 

 glochidia in this genus more extensively than any others and shall describe, as repre- 

 sentative of what has been observed, the hookless glochidium of Lampsilis suhrostrata 

 which is shown in figures 13, 14, and 15, plate viii; and, since it is often necessary in 



the practical work of in- 

 fection to examine the 

 glochidia alive in water 

 and to determine the 

 exact stage of their de- 

 velopment, we shall first 

 speak of their appear- 

 ance when in this con- 

 dition. 



When examined 

 alive (fig. 13, pi. viii), 

 this glochidium exhibits 

 a shell which is compar- 

 atively firm in structure 

 and which may remain 

 unchanged by the water 

 even many days after its 

 living contents have been 

 destroyed. Evidence of 

 the shell's strength is 

 shown by the fact that 

 its shape remains un- 

 changed after the glo- 

 chidial muscle has caused 

 the lips of the shell to 

 bite deeply into a host's 

 tissue, and by the fact 

 that it is not easily broken 

 by rough handling, as 

 when the glochidia are tumbled in and out of a pipette during the process of breaking 

 up the conglutinated masses. This strength is due to the carbonate of lime already 

 laid down in the shell and not to the cuticle, which is often referred to by investiga- 

 tors as though it were the sole constituent of the shell of the glochidium; for when the 

 carbonate of lime is dissolved by acid the cuticle becomes wrinkled and the shell parti- 

 ally collapsed. Viewed from the outside and closed (fig. 13, pi. viii), this shell of the 

 living glochidium exhibits a fine granulation over its entire surface and a distinct border 



Fig. I. — Figures showing relative sizes and shapes of the shells of a series of glochidia, 

 belonging to the following species: A, Sympkynota coTnpIanata. 0.30 X 0.29 mm.; B, 

 5. costata, 0.39 X 0.35 mm.; C. Anodonta cataracla, 0.36 X 0.37 mm.; D, Lampsilis 

 (PropUra) alata, 0.41 X 0.23 mm.; E, Quadrula metanevra. 0.19 X 0.18 mm.; F. Q. 

 pustulosa, 0.30 X 0.23 mm.; G. Plagiola elegans, 0.09 X 0.07s mm.; H, P. securis. 0.31 

 X 0.23 mm.; I, Quadrula ebena, 0.15 X 0.14 nun.; J. Q. plicata. 0.21 X 0.20 mm.; K. 

 Lampsilis gracilis, 0.085 X 0.07s mm.; L, L. recla, 0.24 X 0.20 mm.; M. Dromus 

 dromus, 0.19 X o.io mm.; N, Obliquaria reflexa, 0.23 X 0.225 mm.; O, Unio gibbosus, 

 0.22 X 0.19 mm. 



