The polychæte worms have one or two well-developed parapodia, or limbs, on each side of every segment of the body, and on each [pg173] parapodium is a bunch of bristles, or /chætæ/. There are hundreds of species of this class of worms, the species being based on the shape and the lobes of the parapodia, the relative length of the cirri, the form and arrangement of the chætæ, and so on. They are abundant on the shore everywhere, but abound where rocks and stones afford them some shelter. Many burrow in the sand and mud. In doing this the worm eats his way into the hole, swallowing the mud and sand. He assimilates the organic or vegetable matter the mud contains, and ejects the rest in cylindrical coils, known as castings, which lie in heaps at the mouth of the burrow. By these castings they may be traced. Others secrete tubes. The polychæte worms are divided into /Errantia/, which are wandering, free-swimming forms, and /Sedentaria/ or /Tubicola/, which live in tubes permanently. These distinctions are not strictly correct, since some species of errant polychætes form tubes (/Eunicidæ/), and some /Sedentaria/ form no tubes. Many polychætes are beautifully colored, some in vivid reds and greens, with various markings; some are iridescent; some are phosphorescent. ORDER =ERRANTIA= Carnivorous, free /Polychæta/, with protrusible pharynx armed with a horny jaw. "They are active, fierce beasts of prey." FAMILY ~SYLLIDÆ~ This family is the highest in organization of the worms. They are small, the majority being less than an inch in length, and many are minute. They are often highly colored. Many have long cirri on the feet; some have alternation of generation. They are common everywhere along the coast, but are not seen unless searched for. Sponges sometimes are alive with them. They are interesting subjects for microscopic study. FAMILY ~APHRODITIDÆ~ The scale-bearing annelids. This family of worms is distinguished from all others by having scales on the back. The scales, called /elytra/, are flattened dorsal cirri carried on the upper parapodia, [pg174] and generally on alternate segments and in a double row down the back. The elytra are the breathing-organs, and, although of a horny texture, are richly supplied with nerves. The worms are short and have a large protrusible pharynx armed with a double pair of horny jaws. GENUS ^Polynoë^ The species of /Polynoë/ are determined by the number of scales they bear. They have a flattened short body with nearly parallel sides. The bristles are of a bright golden color. The proboscis is large and has four powerful jaws at the end and a circle of papillæ at the top. They are sluggish in movement and are found under stones and in the crevices of rocks. When disturbed they roll themselves into a ball.