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fine teeth, rise four jointed columns, the last pair of which 
form the door-posts, so to speak, of the gullet. The spaces 
which are necessarily left between these pillars are not filled 
up by any membranous wall, but serve severally as inlets to a 
chamber on either side, which, in that it lodges the gills, may 
conveniently be called “ branchial.” Each branchial chamber 
has further its communication with the world of waters with- 
out carried on or cut off by the opening or shutting of a door 
or lid — the “ operculum ” (op, Fig. 1 ; o, so , i o, p o, Fig. 4)* — 
hinged on behind the eye and put in action by a special set of 
muscles. 
To return for a short time to the pillars just mentioned. 
Each of these is hinged on below — the word will be better 
appreciated presently — to certain of the median chain of 
bones (see Fig. 2) which carry the tongue at their tip ; and 
further consists of a variable number of pieces — at most four — 
also hinged on to one another, which have been termed (pro- 
ceeding from below upwards) hyo - cerato- epi- and pharyngo- 
branchicils (h b,cb, eb, p b , Fig. 2). The two first pair of pillars 
possess all these members, while the third and fourth have a 
hyo-branchial in common ; the pharyngo-branchials of the first 
pair are, further, small and slender bones, while those of the 
succeeding three pairs are broadened out, and are armed on 
their lower surfaces, i.e. those which look downward into the 
pharynx, with fine teeth. 
Since each pair of pillars, through being made up of jointed 
pieces, makes a very appreciable but varying curve with the 
floor and roof of the throat, they have been not inaptly termed 
“ arches.” | The three lower members of each arch further 
carry on their concave (inward-looking) aspect, two rows of 
tooth-like projections (see Fig. 2), which vary much in shape 
and arrangement in the different species of fish ; while on their 
convex (outward-looking) surface they are grooved for the 
lodgment of the gill-plates (Fig. 5), which are arranged, comb- 
like, in a double row on the first three arches, but, in many 
cases, in single series only on the fourth. The foundations, lastly, 
of a rudimentary fifth arch are represented on either side by 
the so-called “ inferior pharyngeal bones ” ( i p , Figs. 2 and 3). 
Before leaving the arches we must notice certain important 
* Lat. operculum , a cover, lid ; from operio , I cover. The same word is 
applied to the door by which the whelk, periwinkle, and other snail-like 
molluscs close the entrance to their shells. 
t For the latest views on the homologies of these branchial arches — a 
most interesting subject, but one which does not come within the scope of 
this article — the reader is referred to a paper by Mr. St. George Mivart, 
F.Tt.S., u On the Vertebrate Skeleton,” published in the last volume of the 
Transactions of the Linnean Society. 
