FRESH- WATER MUSSELS. 175 



Whether or not, therefore, these differences are a true guide to relationships, the 

 gills become one of the most convenient organs for distinguishing genera or species and 

 serve as the most important basis of modem classification. 



Some knowledge of the anatomy of the gills is necessary for proper comprehension 

 of the life process of mussels in breathing, feeding, and reproduction. 



The gills consist, as we have seen, of two platelike bodies on each side between the visceral mass 

 and the mantle. We have thus a right and a left inner gill and a right and a left outer gill. Seen from 

 the surface, each gill presents a delicate double striation, being marked by faint lines running parallel 

 with the long axis and by more pronoimced lines running at right angles to the long axis of the organ. 

 Moreover, each gill is double, being formed of two similar plates, the inner and outer lamellae united 

 with one another below as well as before and behind but free at the top or dorsally. The gill has thus 

 the form of a long and extremely narrow bag open above. Its cavity is subdivided by vertical bars of 

 tissue, the interlamellar junctions, which extend between the two lamellae and divide the intervening 

 space into distinct compartments or water tubes, closed below but freely open along the dorsal edge of the 

 gill. The vertical striation of the gill is due to the fact that each lamella is made up of a mimber of 

 close-set gill filaments; the longitudinal striation, to the circumstance that these filaments are con- 

 nected by horizontal bars, the interfilamentar jimctions. At the thin free, or ventral, edge of the 

 gill the filaments of the two lamellae are continuous with one another, so that each gill has actually a 

 single set of V-shaped filaments, the outer limbs of which go to form the outer lamella, their inner limbs 

 the inner lamella. Between the filaments, and bounded above and below by the interfilamentar 

 junctions, are minute apertures or ostia, which lead from the mantle cavity through a more or less 

 irregular series of cavities into the interior of the water tubes. (After Parker and Haswell.) 



The gills, then, which appear as thin plates, are really comparable to long baskets 

 greatly flattened from side to side, the interior of the basket being subdivided into a 

 series of deep tubes, all in one row. The surface of the basket, which is perforated by 

 many pores visible only with a microscope, is covered with very minute paddles like 

 fine flat hairs. The concerted action of these little paddles, called cilia, keeps driving 

 the water from without the gill through the minute pores into the water tubes. Through 

 these tubes the water passes upward into a chamber above the water tubes, called the 

 suprabranchial chamber, and thence backward and finally out of the shell. 



Since the cilia are habitually driving the water through the surface of the gills 

 into the water tubes, it follows that there must be a regular stream of water entering 

 the mantle chamber from without through the open valves, as well as an outgoing 

 stream passing out from the chamber above the gills. These two streams are known 

 as the inhalent current and the exhalent current, respectively. If a mussel is observed 

 in undisturbed condition on the bottom of an aquarium (PI. V, figs, i and 2), the two 

 openings between the edges of the mantle are readily seen and the currents may easily 

 be observed by introducing with a pipette into the water near each opening a little 

 colored water. The coloring matter placed near the lower inhalent current is drawn 

 into the shell, but that placed near the upper opening is driven forcibly away. The 

 two pronounced currents, or rather two aspects of the same current, are, it may be 

 repeated, formed entirely by the minute paddles surrounding the innumerable pores 

 of the gill surfaces. 



The gills themselves are living strainers in the course of this current, and as the 

 water passes through them the material which serves as food is filtered out to be passed 

 on to the mouth; at the same time, the blood in the minute vessels and spaces within 

 the gill filaments and partitions is being purified and recharged with oxygen. The 

 matter strained from the water becomes clotted with mucus and is driven along by the 

 cilia over the surface of the gills to the labial palpi, where it is taken up and if suitable 

 for food is passed on to the mouth, for the surfaces of the palpi as well as of the gills 



