The week’s biggest gainer was Manhattan Beach casual shoe brand Skechers USA Inc., which shot up 18.2 percent to close at $26.16. Fulgent Genetics Inc. of Temple City also saw big gains, adding 6.8 percent to its stock price to close at $9.88. The week’s biggest laggard was Pasadena’s Arrowhead Pharmaceuticals Inc., which plunged 69.2 percent to close at $1.44.