VARIATION IN PEIIIOSTRACAL APPENDAGES. 
75 
delicate hairs encircliug the whorls. In this state the shell is con- 
sidered to be the Planorhis of Muller. Tlie Vlcipara 
coiitcct(t in its embryonal and youthful stage of growth has three 
principal rows of spirally disposed and 
comparatively long hairs, of which the 
two upper rows are the most strongly 
developed. In addition to these there 
are from twenty to forty intermediate 
spiral ridges bearing much more minute 
and delicate hairs, which rows are always 
most numerous between the base and 
the lowermost row of the stronger hairs, 
and least numerous between the second 
and the third rows. The number also 
appears to increase wdth age, as the 
penultimate whorl on one specimen whicli 
was carefully examined had only five of these finely hispid ridges 
between the suture and the first row of stronger liairs, wliile upon 
the body whorl near the apertuie they had increased to twelve. 
Tlie development of hairs is very variable amongst individuals of 
the same species ; thus the v. Itispidosa of II. /t/.y)ida is a very good 
example of liypertrichosis, being densely covered with stiff recurved 
white hairs, while 
Alder’s v. depilata of 
the same species is 
almost bald ; possibly 
these differences are a 
more or less direct re- 
sult of the environ- 
ment of the two forms. 
These hairs are 
generally caducous, 
cylindrically subulate 
and often borne upon 
a little tubercle or swollen base, and are more especially a 
characteristic of thin and dull coloured species, varying in their 
character according to the species ; those of Helix revehita being 
somewhat short and thick, and those of Helix granidata much longer 
and more slender, with knotty prominences at irregular intervals, 
Fig. 1G8. — Periostracal hairs 
of Helix 7‘cvelata Michaud 
(highly magnified), 
Plymouth, Devon. 
(After an original drawing 
by Mr. G. Sherriff Tye). 
Fig. 1G9. — Periostracal hair.s 
Helix gramtlata Alder, 
lighly magnified), 
Halifa.v, Yorks. 
(After an original drawing 
^ Mr. G. Sherriff Tye). 
Fig. 1G7. — Periostracal hairs or 
processes upon embryonal Vhnpara 
contecta Millet (highly magnified), 
Manchester, 
(.\fter an original drawing by Mr. 
G. Sherriff Tye). 
