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doors are hinged, which is called the carina^ or keel. These 
several parts are held together by a strong membrane, com- 
posed of chitine, a substance which enters largely into the 
composition of the wing-cases {elytra) of beetles, and whose 
main chemical reaction is that it is insoluble in boiling caustic 
potash. 
This is a description of a Carapace — as the whole shelly 
box is called in Crustacean terminology — reduced almost to 
its simplest expression ; but in some pedunculated forms, e.g. 
Pollicipes, there are many more shell elements, all of whicli, 
however, may be reduced to system by comparison with the 
scheme (see fig. 3) laid down by Darwin. Now the shell of 
a sessile Cirripede — e.g. Balanus^ the ‘‘Acorn-shell,” which 
bears a distant resemblance to a miniature jelly-mould — 
though very complex in composition, and having at first sight 
not even the remotest resemblance to that of a Lepas^ will be 
best understood by taking as a key that of Pollicipes^ the 
oldest known genus, “ from which, in one sense, all ordinary 
Cirripedes, both sessile and pedunculate, seem to radiate.” 
If, then, this form be taken as a standard of comparison, the 
homology of the several parts in sessile and pedunculate Cirri- 
pides, according to Darwin, “ admits of no doubt.” These 
relations of homology may, I think, be roughly indicated by 
imagining a pedunculate Cirripede in the position of a stranded 
boat, i.e. keel downwards, cirri consequently uppermost, and 
the Carina and the several accessory factors (such as we have 
in Pollicipes) excluding of course the scuta and terga — to grow 
up and around the animal, encircling it as the outer whorls of 
the perianth do the petaloid stamens of the flower of the 
^ater-lily, while the two pairs of excluded — in a different 
sense mcluded — elements are left to form small trap-doors, as 
it were, for the protection of the upper open end of the trun- 
cated cone thus formed. 
Let us now investigate the internal anatomy of a Cirripede. 
Having taken a Lepas and secured it with pins, keel downwards, 
to a loaded cork, submerged beneath a stratum of spirit or 
water, we shall find our boat covered over at the sterndX half 
by a thick membrane passing from gunwale to gunwale (“oc- 
cludent margins ”) of either scutum^ while at the other end it 
is open for the free play of twelve rowers — the cirri — who 
like galley-slaves sit two on a bench, but far too crowded, up in 
the bows. “ Qu’est-ce done que Ton fait dans cette galere ? ” 
Having carefully divided the membrane which, like a half- 
deck, covers the stern, we come across a strong transverse 
thwart, the adductor scutorum muscle. After severing this 
from one of the points of its attachment, we are enabled to 
