1 82 ROBERT E. COKER AND THADDEUS SURBER. 



inflation. Fig. 7 shows the dorsal aspect of a more advanced 

 specimen; the degree of inflation is apparent; the glochidial 

 shell is insignificant in size as compared with the mussel; it is 

 like a narrow saddle which extends only about half way over the 

 side of the mussel shell. The position of the glochidial shell 

 valves in Figs. 5 and 6 will be better understood from a compari- 

 son with this figure. The specimen of Fig. 6 is viewed from a 

 ventro-lateral aspect. The following measurements (in decimal 

 parts of millimeter) were made from the specimen represented 

 by Fig. 6 : 



Length of glochidium 0.095 mm. 



Width (height) 0.15 mm. 



Length of mussel 0.320 mm. 



Width (height) of mussel 0.215 mm. 



The mussel, as compared with glochidium, is nearly three and 

 one half times as long and nearly one and one half times as wide. 

 It is evident that there must have occurred a material increase 

 in the size of the cellular cyst of the tissue of the host which 

 encloses the young mussel. 



The shape of the mussel in the stage shown by Fig. 7 led to 

 the suspicion that the glochidium was of capax instead of Icevis- 

 simus, but the form of the glochidium is that of l&vissimus, and 

 there is no sign of teeth on the shell. 



It is not known that such a growth in size and alteration of 

 form occurs in any other species during the period of parasitism. 

 What further change in form or growth in size would occur 

 before the liberation of the mussel cannot be determined without 



additional material, not now available. 

 BIOLOGICAL STATION, 

 FAIRPORT, IOWA. 



December 23, 1910. 



