200 BULLETIN OF THE BUREAU OF FISHERIES. 



valves. The glochidial shell and its hooks are still in evidence. In other respects the yotmg mussel 

 shows most of the features which are characteristic of the adult. The anterior end is to the right. 

 Anterior and posterior abductors, anterior and posterior retractors, digestive tract divided into esopha- 

 gus, intestine and stomach with its large diverticula, cerebral, pedal, and visceral ganglia, lithocysts, 

 three gill buds, palp rudiments, the heart and pericardium will be recognized by their resemblance to 

 the adult organs. Sections show the rudiments of the kidneys. From a stained and decalcified speci- 

 men. Actual size, 0.39 by 0.35 mm. 



Fig. 48. Lampsilis ligamentina, a young mussel i week after the close of the parasitic period. The 

 margin of the shell is extended well beyond the glochidial outline and shows the first lines of growth. 

 More calcification has rendered the shell so opaque that the internal organs are no longer visible with- 

 out decalcification. Stained specimens and sections show about the same degree of organization as in 

 the Symphynota larva of figure 47. The foot with its cilia is shown extended and attached to the bottom 

 preparatory to drawing the mussel forward. From a living specimen. Actual size, 23 by 20 mm. 



PLATE XIII. 



Fig. 49. Alasmidonla iruncata. Horizontal section of a water tube of gravid marsupium, taken 

 near ventral border of gill. The respiratory canals (r. c.) are small clefts, indistinctly shown under this 

 magnification (cf. fig. 56}; the marsupial space contains young embryos. 



Fig. 50. Quadrula ebena. Horizontal section of two adjacent water tubes (w. t.) of gravid mar- 

 supium containing young embryos. 



Fig. 51. Anodonta cataracta. Horizontal section of a water tube of gravid marsupium, showing 

 respiratory canals (r. c.) and marsupial space (m. s ); the latter contains yoimg embr>-os. 



Fig. 52. Symphynota complanala. Horizontal section of a water tube of gravid marsupium, show- 

 ing respiratory canals and marsupial space; the latter contains glochidia. Note the thin, stretched 

 interlamellar junctions. 



Fig. 53. Lampsilis ligamentina. Horizontal section of a water tube (w. t.) of gravid marsupium 

 containing young embryos. Note the thin, stretched interlamellar junctions (i. j.). 



Figs. 54-55. Two stages showing process of implantation of a glochidium of Unio complanatus on 

 a filament of a gill excised 2 hours after infection. Figure 54 is taken 3 hours after attachment, while 

 55 is the same glochidium drawn 2 hours later. The greater part of the cyst was formed while the gill 

 was in a watch glass. 



PLATE XIV. 



Fig. 56. Alasmidonla Iruncata. Horizontal section through portion of lamella and water tube 

 of gravid marsupium, showing respiratory canals (r. c.) near ventral border of gill; each canal is sep- 

 arated from the marsupial space by a septum (s). The interlocking cells, forming the suture in the 

 septum, are clearly seen. 



Fig. 57. Anodonta cataracta. Section similar to last, but taken before fusion of folds (s), which 

 are seen not quite touching. The septum is formed by their fusion. Eggs contained in the marsupial 

 space are in an early cleavage stage. 



Fig. 58. Anodonta cataracta. Region marked X in last figure, highly magnified, showing glandular 

 epithelium of respiratory canals (r. c), adjacent blood sinus (b. s.), and histological structure of sur- 

 rounding tissues. Note the muscle fibers. 



PLATE XV. 



Figs. 59-61. Transverse sections of glochidia of Symphynota complanala, taken 15 minutes, 6 hours, 

 and 24 hours, respectively, after attachment to edge of fish's fin, showing three stages in formation of 

 cyst. In 59 proliferation of epidermis is just beginning; in 60 glochidium is about half embedded; 

 while in 61 formation of cyst is completed. In 59, which is more highly magnified than the other two, 

 and in 60 several mitoses are shown in region of proliferation. In 60 cellular detritus from enclosed 

 edge of fin is being ingested by mantle cells of glochidium. 



