. Outlines of zoology. Zoology. ECHINOIDEA. 231 in and out of these pockets, which probably have both respiratory and excretory functions. The free-swimming larva is a Pluteus, very like that of Echinoids (see Fig. 98). Ophiuroids are first found in Silurian strata. Classification.— 1. Euryalida. Skin without plates; arms simple or branched, and capable of being rolled up. Astro- phyton; Gorgono- cephalus. 2. Ophiurida. Skin with plates; arms simple. Oph iopholis, Op hiocoma, Ophiothrix, are common gen- era. Amphiura squa- mata is herma- phrodite. Class ECHINOIDEA. Sea - Urchins, the comm
. Outlines of zoology. Zoology. ECHINOIDEA. 231 in and out of these pockets, which probably have both respiratory and excretory functions. The free-swimming larva is a Pluteus, very like that of Echinoids (see Fig. 98). Ophiuroids are first found in Silurian strata. Classification.— 1. Euryalida. Skin without plates; arms simple or branched, and capable of being rolled up. Astro- phyton; Gorgono- cephalus. 2. Ophiurida. Skin with plates; arms simple. Oph iopholis, Op hiocoma, Ophiothrix, are common gen- era. Amphiura squa- mata is herma- phrodite. Class ECHINOIDEA. Sea - Urchins, the common Echinus edulis, Strongylocentrotus Fig. ioi.—Ventral surface of disc of an Ophiuroid [Ophiothrix fragilis). — After Gegenbaur. jr., Openings of genital pockets or bursae ; 7«., mouth ; ventral plates of arms ; sp., spines of arms ; tube-feet, at the right side these are represented as retracted ; o., the openings through which they are protruded ; p., plates around mouth bearing the so-called teeth ; one of these plates is perforated, and functions as the madreporite. Most sea-urchins live off rocky coasts, and not a few shelter themselves sluggishly in holes. They move by means of their tube-feet and spines, and seem to feed on sea-weeds, and on the organic matter found in mud and other deposits. After the perils of youth are past, the larger forms have few formidable enemies. The hard and prickly body is more or less spherical. The food canal begins in the middle of the lower surface ; it ends at the opposite pole in the middle of an apical disc,. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Thomson, J. Arthur (John Arthur), 1861-1933. Edinburgh, Glasgow and London, H. Frowde and Hodder & Stoughton
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
