144 BULIvETlN OF THE BUREAU OF FISHERIES. 



have observed it on glochidia of the following species : The washboard, Quadrula heros, 

 the blue-point, Q. plicata, the pig-toe, Q. undata, the bullhead, Plcurobema CBsopus, the 

 spike, Unio gibbosus, the slop-bucket, Anodonta corpulenta, and the river pearl mussel, 

 Margaritana margaritijera. The squaw-foot, Sirophitus edentulus, has a modified larval 

 thread (Lefevre and Curtis, 1912, p. 173). 



That the structure of the glochidium is less simple than appears to the ordinary 

 observer is shown by the fact that, in the fully developed glochidium, close microscopic 

 study will reveal the rudiments of foot, mouth, intestine, heart, and other organs which 

 will not, however, assume their destined form and functions until after the period of 

 parasitism. The shell of the glochidium is firm but somewhat brittle owing to the car- 

 bonate of lime of which it is partly composed. If the lime is dissolved out vnth acid, 

 the remaining shell, composed only of cuticle, preserves its general form, although it 

 becomes wrinkled and collapsible. 



The number of glochidia borne in the brood pouches of a fully grown female mussel 

 according to the counts and computations made by various observers, varies in the 

 different species from about 75,000 to 3,000,000. An example of the paper-shell, Lamp- 

 silis gracilis, yielded by computation 2,225,000 glochidia. The mussel was 7.4 cm. 

 (about 3 inches) in length. Several examples of the Lake Pepin mucket yielded glo- 

 chidia in the following numbers, the length of the mussel being indicated in parentheses: 

 (6.1 cm.) 79,000; (7 cm.) 74,000; (7.4 cm.) 125,000; (8.5 cm.) 129,000. 



The glochidia of mussels are very diverse in size and form, although for any given 

 species the dimensions and shape of the glochidium have been regarded as fairly con- 

 stant (Surber, 191 2 and 1915)- Differences in sizes of glockidia within the species are 

 noted by Ortmann (1912 and 1919)" and Howard (1914, p. 8). The matter requires 

 investigation. As regards their form, glochidia are separable into three well-known types : 

 (i) the "hooked" type, (2) the "bookless" or "apron" type, and (3) the "ax-head" type. 



(i) The "hooked" type (PI. XIV, figs, i and 2) possesses a rather long stout hinged 

 hook at the ventral margin of each triangular or shield-shaped valve. These glochidia 

 are usually larger than those of the other two types and the shell is considerably heavier. 

 The hooks are provided with spines which no doubt assist the glochidium in retaining 

 its hold upon the host. As all hooked glochidia generally (though not invariably) attach 

 to the exterior and exposed parts of the fish, the fins and scales, the advantage of the 

 heavier shell and stout hooks may readily be seen. This type of glochidium is possessed 

 by mussels of the genera Anodonta, Strophitus, and Symphynota (floaters, squaw-foot, 

 and white heel-splitter, etc.). (See also text figs. 9 and 12.) 



(2) The shells of glochidia of the "bookless" type (PI. XIV, figs. 3, 4, and 5), while 

 lighter than those of the hooked type, are nevertheless of sufficient strength to with- 

 stand considerable rough handling. So far as we now know, all the glochidia of this 

 type are gill parasites with the exception of the washboard, Quadrula Jieros, which 

 has been successfully carried through the metamorphosis on both gills and fins. The 

 bookless glochidia vary rather widely in shape and in size (text figs. 9 to 12); among 

 the smallest is that of the spectacle-case, Marc/ariVawa monodonia (0.05 by 0.052 mm).; 

 while one of the largest is that of the purple pimple-back, Quadrula granifera (0.290 by 

 0.355 nim.). Placed side by side, about 500 of the smallest or about 80 of the largest 



o Ortmann gives many cases of small discrepancies between his measurements and those of others, based no doubt upon the 

 different sources of material. In several cases he has observed difTerences in sizes of glochidia from different individuals. See 

 papers in the Nautilus, Vol. XXVIII, 1914, and Vol. XXIX, 1915. In one instance he reports glochidia of two sizes from one indi- 

 vidual (1912, p. 353). See also Surber, 1913, p. 4. 



