262 ^"^ ANIMAL KINGDOM 



tions: the end of the pen fits into a cartilaginous groove on the upper 

 side of the head, and two cartihiginous rods at the mantle rim fit into 

 corresponding grooves on the funnel. 



By changing the direction of its funnel the squid can swim forward 

 or backward in the water. When the mantle cavity enlarges, the funnel 

 valve shuts and water is sucked in along the sides of the head. When 

 the mantle constricts, flaps of skin close all openings between mantle 

 and head, and the water is forced out the funnel. The funnel is flexible 

 and is turned backward when the squid wishes to swim head foremost. 

 For the most rapid locomotion, however, the funnel is held straight, 

 pointing forward, and the squid shoots away with its head trailing. For 

 slow movement jet propulsion may be assisted or replaced by the un- 

 dulations of a pair of lateral fins near the apex of the mantle. These can 

 undulate in either direction and in rapid movement are used for steer- 

 ing. 



A spectacular feature of the living squid is its changing color. Just 

 beneath the skin are numerous chromatophores, cells packed with pig- 

 ment that may be black, yellow, or red. WHien a chromatophore is 

 spherical and contracted it is barely visible to the naked eye, but at- 

 tached around its sides are numerous muscle fibers that can stretch it 

 out into a flat disc as much as 3 mm. in diameter. These muscle fibers 

 are controlled by the nervous system, and can act rapidly. A squid can 

 change color in less than a second, or pass waves of color along its 

 body by expanding differently colored sets of chromatophores. 



The arms are covered along their oral surfaces with numerous 

 stalked suckers (Fig. 14.15, A). The arms are relatively short and taper- 

 ing, with the suckers arranged in two longitudinal rows. The tentacles 

 are long, with cylindrical bases and expanded ends having four rows of 

 suckers. If the arms are counted from the upper surface, the tentacles 

 lie between the third and fourth pairs, and can be retracted into 

 pouches formed by fleshy webs between these two pairs of arms. The 

 tentacles are shot out suddenly to capture prey. The arms serve primarily 

 to hold and manipulate the food after it is caught. 



Each sucker (Fig. 14.15 B) is a rigid cup with a finely toothed rim 

 and a flexible bottom attached to a slender stalk. When the tentacle is 

 pressed against a surface the cup is pushed back upon its stalk, obliter- 

 ating the cavity beneath. When the tentacle pulls, the force is trans- 

 mitted through the stalk to the middle of the flexible bottom of the 

 cup, creating suction that holds the cup tight. The squid can release a 

 sucker by contracting small muscles between cup and stalk, pulling in 

 the bottom to eliminate the suction. Thus, the suckers are attached 

 automatically, and can be released only by positive action of the squid 

 unless sufficient external force is applied to overcome the suction. 



Food is shredded by a pair of jaws and a radula similar to those of 

 the nautilus. The slender esophagus (Fig. 14.15 A) traverses the neck 

 to a muscular stomach in the body. Next to the esophagus, at the an- 

 terior end of the stomach, an intestine leads forward to an anus just 

 behind the inner end of the funnel. A very large delicate sac, the 

 caecum, opens into the stomach. Salivary glands open into the esoph- 



