REPRODUCTION AND ARTIFICIAI. PROPAGATION OF FRESH-WATER MUSSELS. 1 45 



III. THE LARVA. 



STRUCTURE OF THE GLOCHIDIUM. 



As has long been known, two well-marked types of glochidia are found in the 

 Unionidae ; one provided with a strong shell bearing a single stout hook at the ventral 

 margin of each triangular valve; the other with no such hooks and a more delicate shell, 

 the valves of which are shaped like the bowl of a very blunt spoon. 



A possible third type, which appears to be a derivative of the second, is seen in the 

 "axe-head" glochidium, originally described and figured by Lea (1858, 1863, and 1874) 

 in Lampsilis (Proptera) alata, loBvissima, and purpurata. 



The first type is characteristically parasitic upon the fins and other external parts of 

 fishes from which scales are absent, the second upon the gill filaments. The occurrence 

 of these types in the genera which we have examined is shown by the following list : 



Hooked glochidia: Hookless glochidia: Axe-head glochidium: 



Anodonta. Cyprogenia. Lampsilis (Proptera) alata. 



Strophitus. Dromus. Lampsilis (Proptera) Isevis- 



Symphynota. Lampsilis (majority of sima. 



species). Lampsilis (Proptera) pur- 



Obliquaria. purata. 



Obovaria. Lampsilis capax. 



Plagiola. 



Pleurobema. 



Quadrula. 



Tritogonia. 



Unio. 



The axe-head glochidium occurs, so far as known, in only a few closely related 

 species which were generally included in the genus Lampsilis, but which, after being 

 first placed in the subgenus Proptera by Simpson (1900), have been elevated to the 

 genus Proptera by Sterki (1895 and 1903), a change which has recently been approved 

 by Ortmann (191 1). The species long known to possess this a.xe-head glochidium are 

 Lampsilis (Proptera) alata, Icevissima, and purpurata, and recently Coker and Surber 

 (191 1) have described it for Lampsilis capax. 



There is considerable diversity in size among glochidia even from the same genus, as 

 represented by the outlines in text figure i (a-o), all of which are drawn to the same scale, 

 the most striking cases being the difference between the two species of Plagiola (g and h), 

 and that between Lampsilis recta and gracilis (k and l). Harms (1909), who has studied 

 the exceedingly minute glochidia of Margaritana margaritijera, finds that they are 

 exclusively gill parasites, because their small size makes attachment elsewhere impossible. 



The type of glochidium is constant for the genus, so far as our observations go, 

 save in the case of Lampsilis, as has just been mentioned. In some cases the shape is 

 also characteristic, as shown by Symphynota and Anodonta (a, b, and c), in which the 

 shell outline is a distinguishing feature. 



In Dromus dromus the glochidium, which is of the bookless type (text fig. i, m), is' 

 greatly elongated antero-posteriorly thus presenting an interesting modification. 



