124 INVERTEBRATE ZOOLOGY 
The head and neck project from the large mantle cavity, into 
which they can be partially withdrawn by means of powerful 
retractor muscles, in very much the same way that a snail’s 
head and foot can be withdrawn into its shell. The siphon or 
funnel, a large funnel-shaped organ at the base of the head, also 
projects from it and can be similarly withdrawn. Gently probe 
the mantle cavity and determine its extent. The mantle con- 
stitutes the outer surface of the body. It will be seen to be a 
cylindrical structure with thick, muscular walls, within which 
lie all the viscera of the animal; its free edge is called the 
collar, as in the snail. It is also necessary to observe that the 
mantle is not a paired organ, as it is in the clam, but an 
unpaired one as in the snail. The squid has no foot, as has 
the clam or the snail, but morphological equivalents of the 
foot are present in the arms and the siphon. 
Since in all mollusks the foot or its equivalent occupies a 
ventral position, and the visceral mass a dorsal position, the 
arms of the squid, together with the head, must be on its 
ventral side, and the opposite end with the broad fins must 
be dorsal; the animal is thus enormously extended dorso- 
ventrally. It will be readily seen also that the mantle falls as 
a cylindrical fold from the dorsal end about the entire body, 
exactly as it does in the case of the snail. In fact, if the 
coils of the snail’s visceral mass could be straightened out, 
the mantle would fall as a cylindrical fold from its dorsal end 
and terminate in the collar below, in the same way as in the 
squid. The morphologically posterior side of the animal is 
that on which the siphon is situated, the anterior side is the 
opposite one. In common parlance, however, the head end 
of the squid is called the forward end, and the fin-bearing 
end, the hinder. The side bearing the fins is likewise called 
the upper side or back, and the opposite side, on which is 
the siphon, the under or lower side. These terms, although 
incorrect in a strictly morphological sense, are much more 
