26 
CENTEAL HALL. 
A small case affixed to the wall at the back of this bay 
illustrates the structure of the Lancelet, by the aid of spirit 
specimens, enlarged models, and coloured diagrams. One of the 
most remarkable features in the structure of this strange and 
primitive little creature is the atrial cavity, enclosing the large 
and complex pharynx. 
Bay VII. Leaving Bay YL, next the principal staircase on the east side 
Mimicry Central Hall, which, as mentioned above, is reserved for 
recently acquired specimens of more than ordinary interest, 
the visitor passes to Bay YII. The wall-cases on both sides of 
this bay are devoted to Molluscs, or " Shell-fish." In the first 
division of the case, on the left wall, are mounted a series of 
specimens in spirit. 
Alongside are a large series of shells arranged on tablets to 
show : firstly, the leading structural types, such as univalve, 
bivalve, multivalve, &c. ; then the nature of the outer layer, or 
periostracu7n ; and, thirdly, some of the more striking styles of 
ornamentation and coloration. Following these, a division is 
devoted to the display of the general form and topography of 
the shells of bivalves, especial attention being directed to the 
nature of the hinge by which the two valves are articulated. 
Freshwater mussels ( Unionidoe) are selected as examples of great 
variability in the form of the shell in closely allied species. 
ISTear by is a small series of the shells of boring bivalves, many 
of them in the substance they perforate. 
The left half of the wall-case on the opposite side of this bay 
shows, in a similar manner, the varieties in form and structure 
presented by univalve gastropod shells. Many of the 
specimens have been cut to show the internal structure. The 
horny, or shelly opercula closing the mouths of many Gastropods, 
are also exhibited in this case. 
The right end of the same case displays spirit preparations 
of the Cephalopod Molluscs, such as Cuttle-fish, Squids, 
Nautilus, Paper-Nautilus, &c. Specimens of the horny 
beaks possessed by all members of this class are also 
exhibited ; and a l^early Nautilus, with the shell bisected in 
order to show the air-cliambers and the comparatively small 
space occu])ied by the "animal," will be found of spocinl 
interest, 
