Fresh-water Mussel. 61 



to which is guarded by the tentaculate postero-inferior portion of 

 the mantle^ representing* the inhalant siphon of the siphonate 

 orders J and a superior^ or anal ehamherj into which a white bristle 

 is introduced in this preparation, and which corresponds to the 

 exhalant siphon of the other orders just named. The large mantle 

 lobes have been turned upwards, and fastened on to the haemal surface 

 of the animal on either side of the raphe, along the middle line of 

 that aspect of the animaVs body. Their thickened muscular border, 

 corresponding to the collar of the Gasteropoda, is well seen, the 

 radiating striae on its lateral surface marking its extent, which 

 corresponded in the living animal to the space between the free 

 edge of the shell and the ' pallial line.' Two main lips run along its 

 free edge, inclosing however between them two somewhat smaller 

 ridges. To segments of this furrowed and ridged surface, portions 

 of the epidermal shell-layer which they secreted are still left ad- 

 hering. The prismatic layer appears to be deposited by the most 

 peripherally-placed strip of the external surface of the organ, just 

 where it becomes continuous with the downward-looking surface 

 just mentioned. The two halves of the dorsal raphe diverge from 

 each other in the genus Unionidae, and then re-unite immediately 

 anteriorly to the posterior adductor. The cirrhi, which guard the 

 entrance to the branchial compartment of the mantle cavity in the 

 natural condition of the animal, are developed from the inner of 

 the two lips of the mantle edge, beginning at a point corresponding 

 with the line of attachment to it of the outer gill lamina. The 

 inner gill lamina of each side fuses, as already said, with its fellow 

 of the opposite side so as to form a continuous floor, perforated only 

 by microscopic apertures, between the anal and branchial chambers. 

 But as the gills fail to become attached to the body for about the 

 posterior three-fifths of its length from before backwards, free 

 intercommunication exists between these two chambers for this 

 space. In this interval between the free gill-edge, along which 

 a large branchial vein is seen to run, and the visceral mass, the 

 glandular portion of the organ of Bojanus comes into view from 

 above. It is from the retia mirabilia of this organ that the branchial 

 inferent vessels take their origin, and pass upwards to distribute 

 themselves on the inner surface of the gill-laminae, whilst the 

 efferent branchial veins have their distribution, as seen by the 

 injection, on the outer or inferior surface of these organs. As shown 



