F. CEPHALOPODA. 387 



At the head the mantle opens by a slit to the exterior, but it can 

 be closed and fastened by various locking contrivances (in Sepia, 

 Loligo, etc., by button-like projections (fig. 385, d] 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, which is fastened to the body and has 

 a wide mantle aperture. Since the cephalopods, by contraction 

 of the mantle wall, can drive the water from the mantle cavity 

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

 the reaction. Here, too, Nautilus is peculiar in that the siphon is 

 throughout life composed of two overlapped folds, which is sig- 

 nificant since in the embryos of other forms the siphon (fig. 396) 

 arises as two separate folds which later unite to produce the defini- 

 tive condition. A typical foot is lacking, but comparative mor- 

 phology shows that the siphon is composed of a pair of epipodia, 

 while the arms are differentiations of fused foot and head. 



Head and trunk are covered with a thin mucous skin, which shows in 

 a marked degree the power of changing color. Loligo will pass from a 

 dark red to a translucent white ; Octopus has an even greater gamut of 

 color. These color changes are possible since in the cutis there is a silvery 



FIG. 386. Female Nautilus, the shell bisected. (From Ludwig-Leunis.) 1, mantle; 2, 

 dorsal lobes ; 3, tentacles : i, head fold ; 5, eye ; 6, siphon ; 7, position of nid- 

 amental gland ; 8, shell muscle ; 9, living chamber ; 10, partitions between 

 chambers ; 11, siphuncle. 



layer over which are numerous different-colored pigment cells or chroma- 

 tophores, in which radial muscle fibres are inserted. On contraction of 

 these the chromatophores are flattened and thus influence the color ; when 

 the fibres relax the pigment cells contract to small spots. In deep-sea 

 cephalopods phosphorescent organs have been observed. 



Notwithstanding the soft bodies a well-developed shell occurs 

 in living cephalopods only in Nautilus and Argonauta (figs. 386, 



