244 INTRODUCTION. 
several times longer and thicker than the inferior pair. 
They are conico-cylindroid in shape, with the free extre- 
mity or point dilated, or bulbous, in the outer side of 
■which the eyes are placed. The inferior pair are short, 
conico-cylindroid, and slightly bulbous at the point. 
The integument is thick at the base of the tentaculne, 
but gradually becomes thinner as it approaches the free 
extremity, where it is delicate and transparent. The 
color is the same on the general investment of the body, 
except at the free extremity of the tentaculae, where, 
from its transparency, the structure beneath shows 
through, and appears whitish. On the superior tenta- 
culie it is rougher than upon the inferior, from the poly- 
gonal folds being deeper. It is but loosely attached to 
the parts within, except at the free extremity, where it 
becomes firmly united. 
The retraction of the tentaculre takes place by means 
of the contraction of the retractor muscle, which forms 
within the tentaculae a cylindrical tube, and is inserted 
into the integument at their free extremity, so that when 
they are retracting the integument becomes inverted, 
and the point of the tentacula first disappears from view 
and, in protrusion, is last to appear. The latter move- 
ment takes place through the relaxation of the retractor 
muscle, and the gradual contraction of the circular, mus- 
cular fibres forming the basis of the integument, com- 
mencing at the base of the tentaculse and proceeding 
towards the free extremity, by which course of movement 
the latter is pushed out. 
