_Pycnogonoïdea or Spider Crabs._ Some of the Spider crabs hook themselves to fishes, while others live under stones, or sprawl with their long hairy legs over sea-weeds, and feed on the gelatinous matter these weeds afford. The throat with its members, and the head soldered to its first ring, forms nearly the whole animal. It has a pair of antennæ and four rudimentary eyes, set on a tubercule. A proboscis-like projection extends from the front; the mouth is furnished with cilia and one pair of foot-jaws. Four pairs of long hairy legs proceed from the throat, spread widely on each side, and end in a hooked claw. The stomach, which occupies the centre of the animal, sends off five pairs of long closed tubes like rays; one pair enters the foot-jaws, the others penetrate the legs. This digesting system is in a state of perpetual vermicular motion, which, as well as the movements of the animal itself, aërate its transparent blood through the skin, by keeping it in circulation. So this insignificant-looking creature has a very curious and complicated mechanism.[39] _Fossil Crustacea._ Analogues to the Anomura are found in the Chalk formation, but the Macrura are the prevailing forms. Extinct species of lobster, crawfish, and shrimps are met with in the secondary strata, from the Chalk to the Coal measures. In the Coal formation all these higher forms disappear, but then the gigantic King Crab, or Limulus, is found accompanied by the minute Entomostracan forms in infinite variety of species. _Epizoa, or Suctorial Crustacea._ The Epizoa infest the skin, eyes, and gills of fishes. Many of them in their adult state bear a strong resemblance to the lowest of the Crustacea; but, in general, the resemblance between these two classes of animals can only be traced during the extraordinary changes which the Epizoa undergo in their early life, and they differ so much in their perfect state that it is wonderful any connection should ever have been discovered between them. The Epizoa are extremely varied in their perfect forms, and the class generally is supposed to be more numerous than the whole race of fishes. In the lower orders of the Epizoa the mouth is suctorial; the higher orders adhere to their victim by jointed mandibles ending in hooks. The Epizoa are male and female: the male is small and free, the female is fixed, and generally has a pair of long egg-sacs hanging from her body. SECTION VIII. CIRRIPEDIA. THE metamorphoses of the Cirripeds, and their resemblance to the lower Crustacea at each moult, are still more remarkable than those of the Epizoa. They form two primary groups, the Balanidæ, or Acorn shells, and the Barnacles or Lepadidæ, which have peduncles or stalks. Both are parasites, but they do not draw their sustenance from the substances they adhere to. [Illustration: Fig. 153. Balanus culcatus.]