18 
J. HOPKINSOX-REMARKS 
The first three of these families comprise the land mollusks 
which are popularly known as “ slugs,” and the last three, those 
known as “ snails,” the most evident distinction between the slugs 
and the snails being that the former do not secrete an external shell 
into which they can withdraw for protection, while the latter have 
such a shell into which they can entirely withdraw. 
It is in the form, construction, and position of the shell that the 
distinction between the different families is most readily perceived. 
In the slugs, the Testacellid^ have a small external shell pro¬ 
tecting their most vital organs, the Limacid2e have an internal shell 
which performs the same function, and the Arioxid^e have an 
internal calcareous plate of indefinite form and loosely-aggregated 
grains in the same position as the shell of the Limacidse. In the 
snails, the Helicid^; secrete a shell, the mouth of which they have 
as a rule the power of closing by an epiphragm, which they can 
break up or dissolve at pleasure, and form again when required; 
while the Cyclostomatid^: close the mouth of their shell when they 
retreat into it by an operculum as permanently attached to them 
as their shell, being merely thrown aside, or opened like a lid, 
when they wish to protrude the anterior portion of their body. 
The Carychiid^ differ chiefly from the Helicidae in their tentacles 
being contractile instead of retractile, and in their eyes being at the 
base of their developed tentacles instead of at the summit. 
It has been objected that too much attention is paid to the shell 
of the testaceous Mollusca, and that their classification should be 
based more on their internal organization, hut it is self-evident that 
all differences in the shell must be due to differences in the animal 
which secretes it and of which it forms an integral part, and that 
therefore the shell is the outward expression or manifestation of the 
internal vital organs. 
The shell is formed by the u mantle,” which is a membranous sac 
enveloping the greater portion of the body and provided at its open 
anterior end, forming the “ collar,” with glands which secrete the 
shelly matter, and it is to these glands of the collar that the coloured 
bands and other markings on the shells are due.*' 
The manner of growth of the shells is peculiar. They remain 
very small until the animal has become dormant for the first time, 
after which they rapidly increase in size. Thus some snails hatched 
in summer increase the size of their shells but little until autumn, 
when they bury themselves in the ground with their heads upwards, 
remaining dormant during winter. In spring they re-appear, soon 
to again bury themselves, but this time with their heads downwards 
* I have here adopted the view that the term “ mantle ” should he applied to 
this organ in both slugs and snails, that part of it which alone, in the snails, 
bears colour-secreting glands being differentiated as the “collar”; by some 
authors, however, this anterior end of the mantle has no distinctive term applied 
to it, and by others the term “mantle” is confined to the organ in the slugs only, 
the “collar” being considered to take its place in the snails. A snail strengthens 
its shell and can repair any part of it by a secretion from the mantle, when the 
glands'of the collar cannot he brought into use, hut such new calcareous matter is 
colourless. 
