284 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [74] 



lar. The interior lateral or basal lobes of the siphon are flexible, and 

 capable of acting as valves to close the opening of the branchial sac by 

 pressing against the inside of the mantle when it contracts. The jet ot 

 water thus forced through the siphon by its reaction propels the ani- 

 mal backward or forward, or in any direction opposite to that in which 

 its flexible extremity may be turned. 



Body varying in form from snbspherical to long-conical. Sides often 

 with fins. Mantle destitute of an external shell. The internal shell, 

 when present, is dorsal,* and may be either horny or calcareous. Sessile 

 arms in four pairs, around the head, provided on the inner surface with 

 suckers or with hooks (modified suckers). Eyes highly developed. 

 Jaws in the form of a sharp, horny beak, the upper jaw shutting into 

 the lower one ; jaws hollow and supported by strong internal cartilages. 

 Odontophore usually with seven (rarely five) rows of sharp teeth. An 

 ink-sac opening near the end of the intestine, at the base of the siphon. 



The exposed surfaces of the body, fins, head, and arms contain within 

 the skin small sacs or vesicles filled with bright-colored fluids of differ- 

 ent colors, but most commonly various shades of purple, brown, red, 

 and yellow. These vesicles are known as chromatophores. They are 

 under the control of muscular fibers, which are so attached to them 

 that, by contracting, they cause the chromatophores to expand into 

 larger, flat, and more or less round spots of color. By the flattening 

 and enlargement of the chromatophores the colored fluids are spread 

 out into thin layers, making them appear of lighter tints. Sometimes 

 the chromatophores overlap each other in several strata when expanded. 

 When their muscular fibers relax the vesicles contract into minute 

 spherical specks, and then appear much darker in color, but are more 

 widely separated, so that the general color is paler. By this means 

 all these animals are able to effect rapid changes in their colors for 

 purposes of concealment, or in accordance with varying conditions of 

 nervous activity. The muscular fibers of the chromatophores are con- 

 trolled by the nerves of the mantle, and contract by reflex action, and 

 also, apparently, in accordance with the will of the creature. Their con- 

 tractility often persists for some time after the death of the animal. 

 When freshly-caught specimens are put into alcohol the chromato- 

 phores expand. 



* In this article, the terms used in describing the form and relations of parts are 

 those in most common use among systematic writers on this group of animals. No 

 attempt is here made to decide the still unsettled questions in regard to the homolo- 

 gies of the arms and siphon with the foot or other parts of Gastropods, nor to apply 

 the later views of Huxley and others as to the general axial relations of the body. 

 For my present purposes I have thought it best to call the oral region the anterior 

 «nd and the opposite extremity the posterior end; when the animal is in its normal 

 horizontal position, the side which is uppermost is called the dorsal side and the lower 

 surface is called the ventral. The prehensile organs are called sessile arms and tenta- 

 cular arms, and tho locomotive tube, is called the siphon, without reference to the 

 homologies of these organs. 



