Fig. 102.—The triangle spider, Hyptiotes sp. (California), with its web; the spider rests on the taut guy-line, with a loop of the line held between its fore and hind legs; when an insect gets into the web the spider loosens the hold of its hind feet on the guy-line, thus allowing the web to spring forward sharply and further entangle the prey. (From Jenkins and Kellogg.)

[CHAPTER XXII]

MOLLUSCA: THE MOLLUSCS

THE FRESH-WATER MUSSEL (Unio sp.)

Structure (fig. [103]).—Technical Note.—The fresh-water or river mussel lives commonly in the streams and lakes or ponds in the United States. It frequents muddy or sandy bottoms. Specimens can often be secured with a long-handled rake from the shore or picked up in shallow streams with the hand. If possible to keep the animals alive until ready for use, some of their habits may be observed. Place them in a tub or trough with water and mud; when they have settled themselves put some powdered carmine, starch, or similar substance in the water near them, and note the water-currents.

Living mussels which have been placed in a dish with mud several inches deep and covered with water will be seen to travel in a definite direction. The end which is in front is the head end. Note the process of thrusting out and retracting the fleshy foot which extends between the two valves of the shell. Note that the two valves are held together along the upper, or dorsal, surface by a horny structure, the hinge-ligament. Note near the hinge-line a prominence (umbo) in each valve from which extends a series of concentric lines of growth. The umbo is the oldest part of the valve. Note at the lower edge of the valves a soft membrane with a fringe along its free border. This is the edge of the mantle-lobes, flaps of the body-wall which cover the body and which aid in the functions of respiration and nutrition.

Technical Note.—Specimens which are to be dissected should be killed by dropping them for a few seconds into warm water, when the muscles will relax enough so that a chip may be thrust between the valves. If specimens are to be kept for some time before dissecting they should be preserved in alcohol or 4% formalin. In a dead specimen carefully remove the left valve. This is accomplished by slipping in a thin knife-blade close to the inner edge of the left valve and carefully cutting the two large adductor muscles which bind the valves together. The dissection should be made under water.