Apparently the earliest cephalopods found diffi- 

 culty in combining tlie activities demanded by preda- 

 tory habits and a carnivorous diet with a slender 

 body concealed in a heavy shell. As an adaptation 

 helpful under these circumstances, they developed 

 the unique ability of secreting gas into the spire of 

 the pointed shell, giving it buoyancy and hence re- 

 ducing the weight to be pulled around. The animal 

 came to live in the enlarged open end of the limy 

 cone, with the gas bubble above it. Addition of lime 

 helped bring the shell into a horizontal position, but 

 at the same time seemingly encouraged elongation 

 of the animal in a direction truly dorsal in terms of 

 its own anatomy. Few animals in the world are so 

 high and so short and so narrow. 



Shell-bearing cephalopods greatly improve their 

 control over the gas bubble by dividing it with trans- 

 verse shell partitions. The location of each of these is 

 evident on the outside of a Spirula shell as a slight 

 constriction, but the nautiluses show no external indi- 

 cations. The shell, in fact, is an object of outstanding 

 beauty, one that appeals especially to mathemati- 

 cians since its curvature (and the curvature of all in- 

 ternal partitions) follows the logarithmic spiral, with 

 each turn of the shell about thrice as broad as the 

 preceding turn. 



Cephalopods are muscular animals, able to subdue 

 struggling prey. They are alert, too, with a far larger 

 and more complexly developed nervous system than 

 is to be found in any other mollusk. Around the en- 

 larged central ganglion above the mouth, they de- 

 velop a good substitute for a skull, formed of cartilage 

 tissue closely resembling that in vertebrate animals. 

 At other points in the body also, cephalopods have 

 cartilaginous rods and bars as stiffening supports. 

 These give the whole animal a firmness that is un- 

 expected from watching the fluid, graceful move- 

 ments of a swimming, darting squid, or the lithe, 

 sinuous flexibility of a live octopus. 



The cynic who remarked that man is the only ani- 

 mal that blushes, or needs to, was not acquainted 

 with cephalopods. Their blushes can put any other 

 creature to shame, and may well be a means of 

 communication. In the thin epidermis over all ex- 

 posed parts of the body are small flexible bags of 

 pigment — blue, green, yellow, brown, or red chro- 

 matophores — each surrounded by a set of radiating 

 muscle fibers. When the fibers contract under the con- 

 trol of the nervous system, the little bag of pigment 

 is suddenly stretched into the form of a flat disk 

 parallel to the surface of the body. Its presence be- 

 comes noticeable as a spot perhaps i j,; of an inch in 

 diameter, whereas when the fibers relax, the pig- 

 ment sac rounds out and becomes invisibly small. 



An excited cephalopod twinkles all over as the 

 pigment sacs change dimensions. Waves of color 

 may sweep along the body, like blushes in a variety of 



hues. At one moment the animal may be conspicu- 

 ously banded and at the next a uniform wine red, and 

 then may blanch uniformly to the ghastly bluish 

 white so characteristic of dead octopus and squid of- 

 fered for sale in fish markets. 



The members of the genus Nautilus (Plate 74) 

 are unique among living cephalopods not only in 

 their handsome shells, often 10 inches across, but 

 also in having about ninety tentacular arms, and 

 twice as many gills (four), heart auricles, and 

 kidneys as any other in the class. The arms, more- 

 over, lack the firm-rimmed, muscular suction cups 

 with which all other cephalopods cling to prey. And 

 the eyes of nautiluses have no lens, making them 

 operate as pinhole cameras — a type of visual organ 

 unique in the animal kingdom. 



All cephalopods except the nautiluses are armed 

 with a gland secreting an intensely dark liquid, used 

 by the animal as an emergency discharge becloud- 

 ing the water and confusing the flavor trail by 

 which an enemy might follow in pursuit. Long ago 

 man began collecting the contents of this gland from 

 cuttlefish in the Indian Ocean for use as a permanent 

 ink — old-style India ink. 



The males of eight-armed octopuses and ten- 

 armed squids can be distinguished from the females 

 by a strange feature of one arm — the lowest on the 

 left side. It differs from the other arms in form and in 

 the shape of the suction cups. The animal uses it to 

 transfer into the mantle cavity of the female the trans- 

 parent bags of sperm cells she needs to fertilize her 

 eggs. 



Fresh-water clams He partly imbedded in sand or 

 mud. with the shells slightly agape and the openings 

 for water currents protruding. (Lynwood M. Chace: 

 National .\udubon ) 



