304 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
appendages richly supplied with bristles or setae; in the fishes the gill 
rakers and in the whalebone whales the fringes of the plates of baleen 
serve the same purpose. Other filter feeders are found among the 
shellfish, such as the oysters, mussels, and clams, and among the lower 
chordates, of which the pelagic forms, such as the salps and appendicu- 
larians, have filters so fine and delicate that they strain out the cocco- 
lithophorids for whose successful capture man must centrifuge the 
water or employ filter paper. 
The structure of the apparatus determines the type of food ingested 
by the filter feeders. The great majority of the copepods, as well as 
the euphausids, have their filters adapted to the capture of phyto- 
plankton. In the plankton-feeding fish there is a nice adjustment 
between the armature of the gill arches constituting the water-strain- 
ing mechanism and their food. First and foremost among the plank- 
ton-feeding fish are the clupeoids, or the herrings, and their relatives, 
the menhaden, sardines, alewives, and shad. Bigelow has remarked 
that the menhaden “has no rival among the fishes of the gulf [of 
Maine] in its utilization of * * * pelagic vegetable pasture; nor 
is any other local species possessed of a filtering apparatus comparable 
to that of the menhaden for fineness and efficiency, though in European 
waters its relative, the sardine, feeds equally on microscopic plankton 
as well as copepods.” As fine-straining as the menhaden’s sieve may 
be, it is unable to retain coccolithophorids. The herring’s gill rakers, 
though they may retain masses of the larger floating algae, seem 
particularly adapted to the capture of copepods, their chief source 
of food. 
The mackerel, with somewhat widely spaced spines on the gill 
rakers of the first gill arch, may at times pick up more or less phyto- 
plankton if composed of fairly large species. Like the herring, the 
mackerel has a sieve more especially suited for the capture of 
copepods. 
Differentiation in filter mechanisms determining feeding habits 
extends also to the various species of whales. The finback and hump- 
back whales, with rather coarse and comblike fringes, feeds largely 
on fish, herring, and sardine, and on the larger zooplankton, particu- 
larly euphausids. The pollock whale, with “unusually fine and curly” 
fringes, almost wholly bristles, and the right whale, with “silk-fine” 
fringes, strain out plankton animals as small as copepods, which 
sometimes are exclusively their food, though at times they feed on 
euphausids either along with the copepods or when the latter are 
unavailable. 
In the whalebone whales, the food-carrying water is taken into 
the mouth and strained as it is forced outward through the fringes 
of the baleen plates by the tongue as the mouth is closed. In the 
