11 



fibre instead of parallel to it as in the resting condition. Each muscle cell is 

 penetrated by a nerve fibre which ends in an irregular arborization near the 

 nucleus. Between ten and thirty muscle cells are attached to the equator of 

 the sheath of the pigment cell , and their contraction transforms the almost 

 invisible spherical pigment mass into a conspicuous colored plate which varies 

 in size from a diameter of less than a millimeter to a diameter of £3 or more 

 millimeters. The chromatophores appear in a definite sequence and arrangement 

 in the embryo , but later this order is obscured or lost. In the adult each of 

 the largest chromatophores , which are somewhat elliptical , is surrounded by a 

 circle of smaller chromatophores. The minute yellow chromatophores are placed 

 between the brown ones. The chromatophores lie at various levels, so that 

 during expansion they overlap one another considerably. New chromatophores 

 are formed throughout life. 



The chromatophores are slightly more numerous upon the upper surface 

 of the animal and are absent from the lower surface of the fins, from the 

 interior of the mantle and siphon and from the portions of the arms which 

 are not ordinarily exposed to light. In other words, excepting the lower 

 surfaces of the fins only those surfaces which are ordinarily exposed to light 

 either from above or below have chromatophores. 



Normally the chromatophores expand and contract in a rythmic manner 

 owing , as Steinach believes , to the tonic action of the suckers. Direct or 

 diffused light causes the chromatophores to expand as long as the suckers are 

 present. In addition to these reflex or tonic causes of expansion or pulsation 

 of the chromatophores, they respond to states of excitement or fear by rapid 

 and more or less simultaneous expansion. Thus under normal conditions the 

 color of the squid is uniformly pinkish brown , but when attacked or attacking 

 or frightened, the squid changes its color with startling rapidity from a pale 

 translucent white to a yellowish white , or pink , or brown color. The animal 

 also assumes at times a mottled brown color. 



The deep fibrous layer is usually thinner than the superficial layer. The 

 two fibrous layers unite wherever the chromatophores are absent. 



The pretty and startling color changes of the squid are heightened by the 

 delicate sheen of the iridiocysts. The layer of iridiocysts usually underlies the 

 chromatophores, but in our species it is beneath the deep fibrous layer except 

 for a few large oval masses of iridiocysts which form scattered spots upon the 

 upper surface of the mantle and fins and which lie under the chromatophores. 

 Each iridiocyst is a much flattened cell whose cytoplasm contains a vast number 

 of sinuous rods which are parallel to the surface of the cell and of the body. 



