34 
Guide to Crustacea. 
Table-case 
No. 3. 
Table-case 
No. 4. 
and “ tergum ” forming the movable lid or “ operculum,” while the 
others form the outer “ wall.” In the genus Catophragmus, how- 
ever, there are numerous additional plates outside those which 
usually form the wall. These outer plates correspond to the 
additional capitular plates found, among the Pedunculata, in Polli- 
cipes, of which a specimen is placed alongside for comparison. 
One of the commonest British Barnacles is the little Balanus 
balanoides which is familiar at the seaside, coating rocks and 
stones as if with “ rough cast.” At the other extreme of size is 
another species of the samq genus, Balanus psittacus, the largest 
member of the sub-class, of which some fine specimens are 
exhibited in Wall-case No. 4. It is found on the coasts of Chile, 
where it is “ universally esteemed as a delicious article of 
food.” 
Several species of sessile Barnacles are commonly found 
attached to large marine animals such as whales and turtles. The 
curious Tubicinella which burrows into the skin of whales is 
exhibited here, and a large cluster of Coronula diadema, growing 
on the skin of a whale, is mounted at the side of the doorway at 
the south end of the gallery. 
Darwin’s Monograph of the Cirripedia, published 1851-1854, is 
still the chief work of reference on this group of animals ; it is of 
special interest to the historian of biological theory, because, in the 
course of its preparation, Darwin had to deal with the problems of 
specific and individual variation as they present themselves to the 
systematic zoologist. Like other groups of sedentary organisms, 
plants and corals for example, the Cirripedia are particularly sub- 
ject to great variation dependent on differences of environment, and 
Darwin often found considerable difficulty in deciding as to the 
limits of species. In Table-case No. 4 is exhibited a small series 
of specimens selected by Darwin himself to illustrate the variations 
of Balanus amphitrite, and accompanied by a list in his hand- 
writing. Of this species Darwin wrote in his Monograph 
“ In order to show that it has not been from indolence that I 
have put so many forms together, I may state that I had already 
named and fully described in detail eight of the following forms as 
species, when I became finally convinced that they were only 
varieties. . . . After studying such varying forms as B. tintin- 
nabulum and amphitrite it is difficult to avoid, in utter despair, 
doubting whether there be such a thing as a distinct species, or at 
least more than half a dozen distinct species in the whole genus 
Balanus .” 
