114: BULLETIN OF THE UNITED STATES FISH COMMISSION. 



meeting-ground for both salt and fresh water forms, with constant additions from each 

 source, resulting in the closest interaction of all, and the consequent thriving of some 

 particular forms. 



Again, the purely physical conditions tend to make these inlets an important 

 swarming-ground for unicellular life, together with the many organisms depending in 

 close connection upon it. The depth of the water is never great, and it is therefore 

 the first to be warmed in the springtime, and is most completely warmed in the summer. 

 The sun's rays penetrate to the bottom and are again, reflected; bottom organisms 

 flourish well, contributing a new class of free swimming larval and adult material. 

 Great banks of eel grass, lying half exposed at low tide to the summer sun, are hot- 

 beds of growth, and harbor an untold complexity of minute organic forms which often 

 cover each blade of the grass with a living slime. The meshes of our gill nets left 

 down during one night were in many cases coated over with the greenish and brownish 

 algous slime gathered by the flowing of one tide through them. These localities are 

 also (according to all the data available at j>resent) the protected retreats into which 

 this species of fish retires at its spawning season in the early summer, and in which 

 the earliest stages of the young are passed. 



These minute organisms furnish directly the food of the menhaden, not only within 

 the limits of these brackish-water inlets and estuaries where the spawn is left to 

 develop, but also wherever the fish is found in the more open coast waters. The whole 

 food supply of this fish is obtained by filtering out from the surface stratum of Water 

 the organic life there suspended. 



The mechanism by which the menhaden secures this character of food is admirably 

 fitted for such a purpose in the high specialization of the "gill-rakers," which are so 

 complete as to render the whole pharyngeal cavity capable of filtering large quantities 

 of water, which the fish takes in — as has often been observed — by swimming actively 

 in circles through the water, with widely-opened mouth and expanded opercula. 



I have given in plates 1 and 2 five figures to illustrate this mechanism. Fig. 1 is 

 an outline (two-thirds natural size) of the adult menhaden in its attitude of swim- 

 ming through the water for food; in the pharyngeal cavity, underneath the opercula, 

 are indicated the positions of the five gill-arches (l y 3, <?, 4, 5) of the right-hand side of 

 the animal; the five corresponding gill-arches of the opposite side of the throat cavity 

 are omitted in the diagram. In fig. 2 are represented the five gill-arches, with all their 

 parts, of a somewhat larger specimen (drawn two-thirds natural size) removed from the 

 fish and placed in order, one behind the other, in a series, of which A is the most ante- 

 rior, the others, B, 0, D, following to E, the small rudimentary one, which is the most 

 posterior. Attached to the axial gill-arches 1 to 5, upon the anterior edge of each, is 

 the row of fine stiff gill-rakers a, b, c, d, e, arranged in a close parallelism like the 

 barbs upon the shaft of the bird's feather, which, indeed, they closely resemble in 

 appearance. These are relatively very loug, reaching far forward upon the inner face 

 of the mouth cavity on the right-hand side, the left side of course having a correspond- 

 ing set attached in like manner to the bony gill-arches as axes. Projecting backwards 

 from these gill-arches are also the regular double-rowed lamellae of branchial filaments 

 m, n, o, p, the respiratory apparatus proper of the tLh; the last gill-arch 5, however, 

 carries no branchial filaments. 



