These six pairs of appendages surround the mouth, and have bristles at the base which serve as jaws. The seventh pair of thoracic appendages are broad plates called the operculum, which meet and cover the abdominal appendages to a certain extent. The abdomen has five pairs of appendages, which are plate-like and resemble the operculum in form and have an upper and an under piece. On the posterior surfaces of the abdominal feet are large, leaf-like folds, which are the gills. In front of the first pair of thoracic legs is a small tubercle supposed to have an olfactory function. /Limulus/ has a blood-vascular system (the blood being bluish in color), a nervous and a digestive system. It walks with the thoracic feet and swims with the abdominal ones. It enters the sand by pushing in the rounded, anterior end. In moulting the carapace opens on the front edge. /Limulus/ is edible, its meat being, it is said, as good as that of the lobster. (Plate LXIV.) CLASS =PANTOPODA= (Lang) ORDER =PYCNOGONIDA= This order of animals, like /Limulus/, has no definitely determined position in the classification of marine organisms. About one hundred and fifty species of /Pycnogonida/ have been described, of which thirty are found on our coast and are commonly known as sea-spiders. They are found crawling slowly over seaweeds, hydroids, and sponges, and appear to be all legs, but, like other animals, have a digestive, a circulatory, and a nervous system. They are devoid of organs of respiration. The body has a cephalothorax, which bears a conical suctorial proboscis on the anterior end, and on top a prominence containing four eyes. It has three pairs of appendages, two pairs of which are sometimes lacking; then come three free segments and a rudimentary abdomen. There is a pair of short appendages bearing claws (chelæ) and four pairs of long walking-legs. The walking-legs contain a tubular outgrowth of the body, into which the stomach extends. The egg-sacs are in the legs, and open at the basal joints. The male is provided with an extra pair of legs, for the purpose of carrying the eggs after they are deposited by the female. The [pg297] ovigerous legs are run through sac-like pockets and bent under the body, and thus the eggs are carried until hatched. [Illustration: PLATE LXIV. Calappa flamma. Calappa flamma, claws open, showing form of cockscomb. Lambrus pourtalesii. Limulus polyphemus.] GENUS ^Phoxichilidium^ ^P. maxillare.^ This species is found on the New England coast creeping over hydroids and ascidians. It is purple, gray, or brown in color. Its young take shelter in the cavities of hydroids, forming galls. [Illustration: /Phoxichilidium maxillare/, enlarged.] VII MOLLUSCA TABLE SHOWING THE CLASSIFICATION OF THE MOLLUSKS DESCRIBED IN THIS CHAPTER Phylum =MOLLUSCA=