118 COMPARATIVE ANATOMY CHAP. 



In many Octopoda, the long arms are connected by means of membranes near 

 their bases, and occasionally as far as their tips. In the latter case the circle of 

 arms has the appearance of an umbrella, of which the arms are the ribs. The 

 mouth lies in the centre. The Octopoda can creep by means of their circle of arms, 

 the visceral dome standing erect. In this position they may best be compared 

 with snails, the ventral side of the circle of arms functioning like the sole of the 

 snail's foot. 



The Decapoda have ten arms ; eight of these correspond with the eight arms of 

 the Octopoda, but are shorter and are never connected by membranes. The two 

 others, the prehensile tentacles, are inserted between the third and fourth Octopodan 

 arms on each side and differ from the latter in structure, being long and vermiform, 

 with swollen ends armed with suckers, hooks, etc. The prehensile tentacles are 

 very contractile, and in many Decapoda (e. g. Sepia) are concealed in special cavities 

 of the head when the animal is at rest. These cavities probably correspond morpho- 

 logically with the water pores, which often occur elsewhere at the bases of the arms 

 or on the head. When pursuing prey the Decapods dart these tentacles out of their 

 cavities with great force. 



One (less frequently two) of the eight or ten arms of the male Dibranchia is 

 almost always transformed (hectocotylised) to assist in copulation. In some 

 Octopoda it even becomes detached from the body and is regenerated. 



The hectocotylised arm is, in the Octopoda, usually the third arm on the right 

 side, and in the Decapoda the fourth on the left. (The arms are counted from 

 before backward.) 



In the female Argonaut, each arm of the first pair is widened into a sail-like 

 expansion, which stretches back over the outer surface of the shell. 



All Cephalopods, even the more massive Octopoda, are good swimmers. In swim- 

 ming, the mantle and funnel play the chief parts. "Water is alternately taken into 

 the mantle cavity through the mantle cleft, and expelled from it forcibly through 

 the funnel, the reaction propelling the animal backwards. When the water is being 

 ejected, the mantle cleft is closed by the locking apparatus, so that all the water in 

 the mantle cavity has to pass out through the funnel. Many Decapoda can also 

 swim with the head directed forward, the lower (distal) end of the funnel being bent 

 round, so that the water is expelled in the direction of the visceral dome. In 

 swimming the arms are apposed to one another, so as to diminish the friction as 

 much as possible. Some Octopoda, especially those which have interbrachial 

 membranes, assist themselves in swimming by opening and shutting their circle of 

 arms like an umbrella. 



XI. Swelling 1 of the Foot (Turgescence). 



Imbibition of Water. 



The foot in many Lamellibranchia and Gastropoda may swell when 

 it has to be protruded from the shell and used for locomotion. Until 

 recently opinions varied very much as to the way in which this swell- 

 ing or expansion took place. Many believed that water was taken 

 up from without into the blood vascular system or into a special 

 water vascular system, but there was difference of opinion as to the 

 manner in which it was taken in. On the one hand it was said to 

 enter through apertures or pores in the foot, which, however, do not 

 exist, the only pores found being the apertures of the pedal glands 



