MOLLUSCS. 



365 



FlO. 478. — Chromato- 

 phore of squid, ex- 

 panded and con- 

 tracted; enlarged. 



Among the peculiar features of the cephalopods should be mentioned the chrom- 

 atophores, by which the animal is enabled to change its color quickly. Such structures 

 are found elsewhere in the animal kingdom, but here they attain 

 the greatest development. They are best studied in the young 

 just after hatching, as then the whole animal can be placed under 

 the microscope and examined with comparatively high powers. 

 Each chromatophore is in reality a cell filled with pigment, and 

 expanded by suitable muscles attached to its wall. These cells 

 are placed just beneath the epidermis, and, when expanded, the 

 contiguous ones nearly touch, so that the animal appears of a uniform tint. When 

 the chromatophore contracts, as it does by its own elasticity, the color is condensed 

 so that it appears as a very minute black dot, and then the translucent, almost 

 transparent, character of the tissues is seen. The expansion and contractions of 

 the chromatophores take place with considerable rapidity, so that the changes in 

 color are remarkable. There are several sets of these chromatophores, each dis- 

 tinguished by its color, yellow, red, blue, or brown ; and each set expands independ- 

 ently of the others, so that an animal almost transparent will suddenly turn bright red, 

 instantly change to a blue, and then a yellow or a brown, or back to its previous colorless 

 condition. The chromatophores in different parts of the body are also independent, 

 and sometimes the fins will be bright blue and the anterior part of the mantle an 

 equally vivid red. When examined under the microscope the whole of these changes 

 can be carefully studied, and then it is found that the chromatophores are suddenly 

 expanded, but that their contraction is accomplished in a more leisurely manner. 

 When expanded they have a diameter of about a fiftieth of an inch, but when reduced 

 to their smallest size they measure but one two hundred and fiftieth of an inch. The 

 purpose of this change of color is protective ; and as by it the animal can assimilate its 

 color to any surroundings, it can readily escape observation by any fishes on which it 

 may wish to feed or which might otherwise feed on it. In the Nautilus these chro- 

 matophores do not appear to be present, though they exist in all other forms. With 

 this capacity for change it will readily be seen that color is a character of no impor- 

 tance in disci-iminating the species of cephalopods. 



The sexes of the cephalopods are separate. Of the internal 

 reproductive organs we need say nothing, but in connection 

 with the reproductive act there are some very interesting fear 

 tures. Many years ago Cuvier in dissecting a female Para- 

 sira, found attached to it a peculiar worm-like object furnished 

 with numerous suckers. He regarded it as a parasite, and gave 

 to it the name Sectocotylus. At about the same time the 

 Italian naturalist Delle Chiaje found a similar structure in 

 Argonauta, which he called Trichocephalus. Several years 

 elapsed before the true nature of these structures was recog- 

 nized. It was then discovered that this supposed parasitic 

 worm was in reality one of the arms of the male, which sep- 

 arates and serves to convey the male reproductive element 

 to the female. This ' hectocotylization ' reaches its highest development in the two 

 genera named above. In these, one of the third pair of arms (right or left, according 

 to the genus) becomes thus modified. Just before the period of reproduction, this 

 arm .appears as a nearly globular sac. This then bursts, and from it issues an arm 



FtG. 479.— Heotocotylized arm 

 of Arganauta after separa^ 

 tiou from the male. 



