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 subspherical 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 saes or vesicles filled with bright-colored fluids of differ- 
ent colors, but most commonly various shades of purple, brown, red, 
and yeliow. 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 willof 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 
end 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 the locomotive tube,is called the siphon, without reference to the 
homologies of these organs. 
