PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE, 
195 
the last whorl was bent abruptly to one side, and quite de¬ 
tached from the previous whorl. 
Dr Buchanan White exhibited a large spider (Mygale) 
from Cuba, presented to the Index Collection by Mr H. 
Crawford, Kinvaid. Some specimens of Mygale, he said, 
attained a much larger size, and were able to catch and kill 
small birds. 
The following paper was read 
“Shells: Their Structure, Growth, and Uses." By Mr 
H. Coates, F.K.P.S. 
I cannot commence this paper on shells without 
referring to the loss which science has sustained in the 
death of Dr J. Gwyn Jeffreys, who passed away within 
the last few days, at the ripe age of 76. Dr Jeffreys 
has spent almost a life-time in throwing light on the 
habits and distribution of the mollusca of our coasts, and 
of the neighbouring seas, and his British Conchology, in 
five volumes, is the most important and comprehensive 
work we have on the subject. He was one of the most 
constant attendants at the meetings of the British Associa¬ 
tion, and conducted several dredging expeditions organised 
by its Committee. With him the study of natural history 
was pre-eminently a labour of love. In proof of this I 
need ©nly quote one sentence from the work above referred 
to :—‘‘The study of our native mollusca has been to me 
from childhood such an inexhaustible source of pleasant 
and innocent occupation, it has given me so many happy 
hours, and it has taken away or alleviated the sting of so 
many sorrows, that I am desirous to assist in making it 
more an object of general cultivation than it has hitherto 
been.’’ 
In former papers I have given some account of the ana¬ 
tomy and physiology of the two great classes of the 
mollusca which are represented by our local land and 
fresh-water shells. In these papers the shell itself came 
in for a share of description along with the other parts of 
the organism, but necessarily only a very brief and 
partial description. My present object is to look a little 
more closely into this the most popular part of molluscan 
anatomy. Popular the subject certainly is, and yet I 
trust we shall find that shells are not merely attractive to 
the casual observer, but offer to the student of Nature 
some of the most curious and interesting problems with 
which he has to deal. Without a knowledge of the 
creatures that formed them, and of the way in which they 
were formed, the mere shell-fancying which has been 
dignified with the name of “conchology” can lead to no 
results beyond the accumulation of a cabinet-full of objects 
which are curious or rare, and nothing more; but with this 
knowledge, each specimen has a lesson to teach us. 
Without further introduction, let us ask ourselves what 
a shell is. The question seems a simple one, and yet it is 
by no means easy to answer. It is part of the work of the 
comparative anatomist, in studying any particular organ, 
to endeavour to discover its relationship to similar organs 
in forms of a different type, both as regards its structure 
and its use; in other words, to find out its homology and 
its analogy. Let us try, therefore, to define the shell of a 
mollusk in terms of this two-fold relation. As regards its 
homology, the definition must be negative rather than 
positive, for a shell is one of those structures which do not 
seem quite to correspond with anything we find in either 
higher or lower types of the animal kingdom. The most 
we can ,say is that, as a skin-secretion, it corresponds to 
such appendages of the skin as hairs and feathers. At 
first sight it might appear to be homologous with the 
shelly covering of a crab or lobster, but the differences of 
type and structure are wide, as we shall presently see. 
The analogies of the shell are much more obvious. Here 
we have the true correspondence with the armour of the 
crab, as also with the limey framework of other inverte¬ 
brates, whether external or internal. The analogy be¬ 
tween a shell and the skeleton of a vertebrate is marked, 
as both serve not only as supports for the soft parts of the 
body, but as mechanical means of attachment and leverage 
for the muscles. Thus, as we can bring our finger and 
thumb together by means of the muscles which are 
attached to the bones of these limbs, so the oyster can 
close its shell by means of the muscle which connects the 
valves. For this reason, therefore, the shell may be de¬ 
scribed in general terms as the exo-skeleton of the mollusk. 
When, however, we come to compare the shells of different 
groups of mollusca, we find that it has further a special 
end to serve, namely, to protect the breathing organs and 
heart. One proof of this is that the most rudimentary 
shells, namely, those of the slugs, are in general merely 
thin plates of limey material covering these organs; and in 
one slug, the testacella, where the breathing organs are 
removed to the extremity of the body, the shell is still 
found associated with them. The development of the 
shell also is closely connected with that of the heart and 
breathing organs. For these reasons the shell has been 
called a pneumo-skeleton. There is one exception to this 
relation, namely, in the “ paper nautilus,” but the struc¬ 
ture and origin of that beautiful shell are altogether so 
abnormal that it need hardly be taken into account. 
We are now perhaps in a. better position to answer our 
