MOLLUSKS 155 
only as a pouch to contain the eggs while they are being carried 
about by the female. When first taken from the water it is soft and 
flexible, but soon hardens and becomes very brittle in the air. When 
much disturbed the female may cast it off, and it is never developed 
by the male argonaut. 
In all forms excepting the chambered nautilus the body is 
cone-shaped or dome-like, and is enclosed in a mantle which is at- 
tached along the back, and hangs freely around the body, encasing 
it as a bag on the sides. The head and tentacles project beyond the 
mouth of the mantle-bag, as does also the siphon. This siphon is a 
tube which is developed on the side of the head, and is open at both 
ends. In the adult nautilus and in all embryonic Cephalopods this 
siphon is composed of two side flaps whose edges fold together to 
form a tube, but in all adult cephalopods, excepting nautilus, the 
edges fuse forming a solid-walled tube. 
When the mantle-bag expands, water is drawn in through the 
slit between the body and mantle on both sides of the neck. I the 
animal be quietly breathing, the water is forced out through the 
same slit by the contraction of the mantle, but if the cephalopod 
desires to move, the free edge of the mantle-bag becomes hooked to 
the outer wall of the siphon, thus closing the neck-slit so that the 
water is driven forcibly out through the tube of the siphon. This 
siphon tube usually projects forward, and the concentrated stream of 
water rushing out of it forces the animal backward with great 
rapidity. The siphon can, however, at times be directed backward, 
thus driving the animal forward, but this is so rarely done that it 
‘an not be called the usual manner of progression. 
A careful study of the subject has convinced naturalists that 
the siphon has been derived from what was the middle part of the 
foot, in the extinct mollusks from which the Cephalopods have de- 
scended. 
The so-called ‘side fins’? and “tail” of the squid are mere 
expansions of the mantle. It is interesting to observe that the so- 
salled tail fin is not at the posterior end of the body but at the 
highest point of the back of the animal. The sucker bearing arms 
being derived from the fore part, and the siphon from the middle 
part of the foot, while the conical body has been elongated dorsal- 
wards. These things can, however, only be fully comprehended 
