34 Guide to Cnisiacca. 



Table-case and " tei'gum " forming the movable lid or " operculum," while the 

 ■'^°' ^" others form the outer " wall." In the genus Catophragmns, how- 

 ever, thei'e 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 PoIIi- 

 cipcs, of which a specimen is placed alongside for comparison. 



One of the commonest British Barnacles is the little Balaniis 

 halanoidcs 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 same genus, Balanus psittacus, the largest 

 member of the sub-class, of which some line 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." 

 Table-case Several spacies of sessile Barnacles are commonly found 

 ■ attached to large marine animals such as whales and turtles. The 

 curious Tiihicinella 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 amphitritc, 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- 

 nahulmn 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^ 



