V. CEPHALOPODA 



339 



ventral incision (fig. 351) the two ctenidia (four in NauliUts) are seen on 

 the sides. Between them, in the middle line, is the anus, and right and 

 left of this and a little behind are the nephridial openings (four in Nau- 

 tilus, which also has osphradia). More lateral are the sexual openings 

 of which one (usually the right) is commonly aljsent. At the head the 

 mantle opens Ijy a transverse slit to the exterior, but it can be closed and 

 fastened by various locking contrivances (in Sepia, Loligo, etc., Ijy button- 

 like projections (</) which fit into corresponding sockets {b) on the trunk). 

 When thus closed the communication with the exterior is by a special 

 conical muscular tube, the funnel or siphon (Tr.), which is fastened to the 

 body and opens widely to the mantle cavity. Since the cephalopods, by 

 contraction of the mantle wall, can drive the water from the mantle ca\'ity 

 through the siphon with great force, they can swim very rapidly by the 



Fig. 352. — Female Nautilus, the shell bisected (from Ludwig-Leunis). i, 

 mantle; 2, dorsal lobes; 3, tentacles; 4, head fold; 5, eye; 6, siphon; 7, position of 

 nidamental gland; 8, shell muscle; 9, h^ng chamber; 10, partitions between chambers; 

 II, siphuncle. 



reaction. Nautilus is peculiar in that throughout life the siphon is com- 

 posed of two overlapped folds, which is significant since in the embryos 

 of other forms the siphon (fig. 361) arises as two separate folds which 

 later unite to produce the definitive condition. A typical foot is lacking, 

 but comparative morphology shows that the siphon is composed of a pair of 

 epipodia, while many zoologists regard the arms as differentiations of the 

 fused foot and head, since they are innervated from the pedal ganglia. 



Head and trunk are covered with a thin mucous skin, which has the power 

 of changing color in a marked degree. Loligo will quickly pass from a dark 

 red to a translucent white; Octipus has an even greater gamut of color. These 

 color changes are possible since in the corium there is a silvery layer over which 

 are numerous different-colored pigment cells or cliromatiph-tres, in which radial 

 muscle fibres are inserted. On contraction of these tlie chromatophores are 



