REPRODUCTION AND ARTIFICIAIv PROPAGATION OF FRESH-WATER MUSSELS. 171 



epithelial cells are undergoing degeneration, while on the left of the section quite a patch 

 of these cells is sloughing off, a not infrequent occurrence. The region of most active 

 growth and multiplication of cells is just below the line of constriction, and, as the cells 

 at this level increase in number, they appear to push those lying above them up over the 

 outside of the shell, so that the actual covering of the glochidium is due largely to this 

 mechanical gliding of the epithelium over its surface. Sections give no conclusive evi- 

 dence of amitotic division, while mitoses are generally abundant in the region of active 

 proliferation. An intermediate step in the process of implantation is illustrated in 

 figure 60, plate xv, less highly magnified than the last figure, which shows a glochidium 

 about half covered in six hours after attachment. The free edges of the cyst wall even- 

 tually meet over the dorsal side of the glochidium, where they then fuse. Figure 61, 

 plate XV, shows a case of complete implantation on a fin at the end of 24 hours; now the 

 epithelial covering is continuous and the glochidium entirely inclosed. The wall of the 

 cyst is seen at this time to be quite thick, but it usually becomes thinner later on as the 

 cells composing it flatten down. In the last two figures the mantle cells of the larva 

 clearly show epithelial nuclei and cell detritus which have been ingested. 



In figures 62 and 63, plate xv, two stages are represented in the formation of the 

 cyst on gill filaments, taken at one hour and three hours, respectively, after attachment. 

 The glochidia are those of Lampsilis ligamentina . In figure 62, plate xv, the prolifera- 

 tion has made some progress, especially on one side, and three or four mitotic figures are 

 seen just below the glochidium and near the raw edge of the constricted epithelium. 

 A large mass of the tissues of the filament is also shown in the figure inclosed within the 

 mantle chamber of the glochidium. Figure 63, plate xv, represents a stage when the 

 process is nearly completed and the edges of the epithelial covering have met but not 

 yet quite fused. The cyst wall in this case is much thinner than that shown in figure 

 61, plate XV, but its thickness is quite variable. 



In about one week after attachment, as a rule, the wall of the cyst begins to assume 

 a looser texture, the intercellular spaces becoming infiltrated with lymph, and from 

 this time on to the end of the parasitic period there is Httle further change in its 

 structure. 



Before liberation of the young mussel, the valves open from time to time and the 

 foot is extended. By the movements of the latter the cyst is eventually ruptured, its 

 walls gradually slough away, and the mussel thus freed falls to the bottom. 



Portions of the wall of the cyst often adhere to the shell after Hberation, while, if 

 the young mussel has hooks, it may hang for a time by shreds of the fin in which the hooks 

 are embedded, as seen in figure 24, plate ix. 



METAMORPHOSIS WITHOUT PARASITISM IN STROPHITUS. 



In a brief paper (191 1) we have recently announced the discovery that in the genus 

 SlrophHiis Rafinesque the metamorphosis takes place in the entire absence of parasitism, 

 and, since the life history of this form is without a parallel in the Unionidae, so far as is 

 known, reference may be made again to the interesting conditions which obtain in its 

 development. 



