TIIK MUSSEL. 65 



PLATE XV. 



THE FRESH-WATER MUSSEL. THR EXTERNAL CHARACTERS AND THE SHELL. THE 

 ALIMENTARY AND EXCRETORY ORGANS. THE HEART, ETC. 



FIG. I. The living animal drawn from the left side, as seen in a deep, laterally com- 

 pressed glass vessel half filled with sand, the surface level of which is represented by a 

 transverse line. 



The arrows indicate the course of the induced ciliary currents. 



FIG. II. The same, seen from behind.* 



FIG. III. The right valve, from within. 



If carefully dissected from the body under water, it will be seen that the superficial 

 chitinous layer is continuous on the haemal side, between the points i. n., and also for 

 the area marked s'., where it is turned in and reflected on to the pallial muscle. (Compare 

 the transverse sections figured on Plate XVI.) 



FIG. IV. Dissection from the side, the left valve and pallial lobe alone removed. 



The cut edge of the mantle is specially shown, as it indicates the line of attachment of 

 the gills and labial palps, and consequently that of demarcation between the supra and infra- 

 branchial chambers. 



In the specimen figured the external gill lamina was fully distended with embryos. 



FIG. V. The same, dissected to the level of the alimentary canal ;f this, the pericar- 

 dium, heart, and aortae, have all been opened up. 



The labial palps were cut down to the level of the middle line. 



In this figure the suprabranchral chamber, incompletely subdivided by the suspensory 

 ligament of the branchiae, is also seen (compare Figs. VII. and VIII. of Plate XVI.) 



(Figs. I. to V. all nat. size.) 



FIG. VI. The excretory organ, dissected from the left side, shortly after death. 



The left half of the mantle-lobe was first removed as for Fig. IV., and after that the 

 greater part of the outer, and a small portion of the outer wall of the inner gill laminae, as 

 indicated by their cut ends drawn. The genital and excretory apertures being thus exposed, 



* The larger tentacles of the inhalent siphon are undoubtedly sensory, and I have witnessed the ejection of embryos 

 through the pore marked *, first figured and described by Keber (70). Compare Figs. IV. and V. 



f If any difficulty is experienced in following tho coils of the alimentary canal, it can bo overcomo by first injecting 

 with plaster of Paris. This method should, however, be used with caution, as the parts are liable to be unnaturally- 

 distended. 



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