74 
VARIATION IN PERIOSTRACAL APPENDAGES. 
1 1 . J 
* 4 M 
./ 44 ^‘ 
Fig. IGF — Periostracum of Helix 
rex^clata Midi, showing the arrange- 
ment of the hairs upon its surface 
(highly magnified). 
leaves or logs, etc. ; the luollusks fre(iuentiiig such situations usually 
possess shells of a dull and horny tint, 
uithont the brightly-coloured markings 
distinguishing species living a more e.\- 
posed and prominent life. The epidermis 
is also more than ordinarily thick and 
perceptible, and is in some species pro- 
duced or continued into delicate hairs 
or bristles, the depressed or flat spired 
species often possessing hairs of com- 
paratively greater length than those 
species whose whorls are more conically coiled. The hairs are usually 
distributed over the surface of the shell in a more or less regular and 
symmetrical manner, and being developed by processes of the mantle 
margin and not produced at random, they 
often have a perceptibly definite arrange- 
ment in the different .species. 
Accoi’diug to iMr. W. .Jeffery, these hairy 
processes are formed as thick mucus on the 
surface of the mantle parallel with the 
plane of the whorls and are afterwards 
elevated to their perpendicular position by 
the succeeding calcareous layers. 
Tye has observed that these hairy appendages in the terrestrial 
snails are hygrostatic in character, and 
- fr'A > 
Fig. 1G). — A portion of one 
of the spiral ridge.s of the ])eri- 
oj'tracumof I'lanorlis alius var. 
(lra/>arnau(li (Shepp.) showing 
the hairs and their arrangement 
upon the. ridge (after a highly 
magnified original drawing by 
Mr. G. SherrilT Tye). 
become erect and cons})icuous during dami) 
fe-.,;] • , 'll,!); 
or moist weather, such as these mollusks 
are most active in, the hairs then forming 
a chei'ein'-de-frhe, probably very repellant 
to most creatures disposed to prey upon 
these mollusks, and thus may well be pro- 
tective. The protective character of these 
periostracal or epidermal ai)pendages is 
sui)ported b}" the fact that many of our 
larger freshwater as well as terrestrial si)ecies, like the Vivijxirw, 
Plduorh 'ts conieus, llelir aintiaxa, etc., are in their young and more 
helpless stage quite hispid, and thus reap such iwotection as this 
defence may afford. The hispid surface of the young Fldiiurhis 
Cornells is composed of twenty-five to thirty spiral rows of short 
Fig. IGG. — Pla/toilis conieiis 
(Iv.) 1") days old, X 12, 
Showing ihc si)iral rows of 
periostracal hairs characteristic 
of the youthful stage, 
(After an original drawing by 
Mr. (T Sherrifi' Tye). 
