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- 

 tach.ed 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. If the 

 animal be quietly breathing, the water is forced out through the 

 same slit by the contraction of the mantle, biit 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 forcil^ly 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 Avith great 

 rapidity. The siphon can, however, at times be directed backward, 

 thus driving the animal forward, but this is so rarely done that it 

 can not be called the usua^ 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- 

 called 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 



