150 BULLETIN OF THE BUREAU OF FISHERIES. 



over a period of four or five minutes. During this suspension in the water the sucking 

 of a pipette will draw in glochidia over a wide area, as they are pulled by the invisible 

 strands into which the mucus has been divided. The significance of this mucus and the 

 absence of the thread gland are discussed under another heading of this paper. The 

 mucus is dissolved by the water in a short time, so that after 24 hours the glochidia are 

 found entirely free and snapping actively upon the bottom. We find that these glochidia 

 can be freed from the mucus by repeated washing, and that it is desirable to do this at 

 once if one wishes to keep them alive for the maximum period. When thus set aside 

 it is possible for them to remain ahve for as long a time as two or three weeks. 



In killing this glochidium we have used successfully crystals of chloral hydrate or 

 hydrochlorate of cocaine added to the water of a watch glass containing the glochidia, 

 and fixation with Merkel's fluid, or with weak corrosive sublimate, as described for 

 the bookless type. 



Stained specimens show the same rudiments of stomodaeum, enteron, and meso- 

 dermal structures, as described by Lillie (1895) and Harms (1909) for the glochidium of 

 Anodonta. The lateral pits are conspicuous and the cells of the larval mantle are well 

 developed laterally, though thinning out over the median part of the larval adductor, 

 where their boundaries are not clear and only a few nuclei are discernible. Sections show 

 two kinds of granules within the larval mantle cells, one staining deeply with iron 

 haematoxylin and the other with acid-fuchsin. Near each corner of each valve is a cell 

 which stains deeper than the rest and seems to contain more of the granules. The 

 significance of these six cells we can not determine. The sensory cells (fig. 9, pi. viii) 

 are sHghtly different in position from those in Anodonta. Lying along a line drawn 

 across from hook to hook are three large cells in line beneath the hooks and a smaller 

 one on either side between the larval adductor and the lateral pit. 



THE PROPTERA OR AXE-HEAD TYPE. 



This glochidium possesses hooks which are not homologous with those of the 

 Anodonta type and is to be regarded as more nearly related to the hookless forms, an 

 interpretation which is borne out by the fact that the "axe-head " can be readily imagined 

 as a modification of the glochidial outline seen in some species of Lampsilis, the glochidia 

 of which, like those of subrostrata (fig. 13, pi. viii), show some approach to a rectangular 

 form. Its four hooks are so arranged that those of one valve pass inside the opposite 

 ones, thus bringing the ventral margins close together and giving a very firm hold upon 

 the host's tissue. In other respects it does not show marked differences from the hookless 

 type, and the few experiments we have made with it indicate its attachment to the gills 

 rather than to the fins. 



Recently Coker and Surber (191 1) have observed "an almost exactly similar 

 glochidium" in Lampsilis capax, while in Lampsilis (Proptcra) larvissima they find an 

 axe-head glochidium which is of a somewhat different outline and lacks the hooks. 

 They point out the fact that in Lampsilis gracilis, a species which in its adult features 

 (form of shell) seems almost to intergrade with lavissimn,, the glochidium is of the ordinary 



