. Outlines of zoology. Zoology. 224 ECHINODERMA. sequence there is frequently a very complete skeleton. From the primitive gut of the larva, pouches grow out to form the usually spacious coalom and the characteristic ivater vascular system, which may have locomotor or respiratory functions or both. The branches of this system, together with the nerves, exhibit in most cases a typical five-rayed arrangement. In addition to the water vascular system, there is a system of bloodvessels. Well-defined excretory organs are absent. The sexes are almost always separate. There is usually a striking circ
. Outlines of zoology. Zoology. 224 ECHINODERMA. sequence there is frequently a very complete skeleton. From the primitive gut of the larva, pouches grow out to form the usually spacious coalom and the characteristic ivater vascular system, which may have locomotor or respiratory functions or both. The branches of this system, together with the nerves, exhibit in most cases a typical five-rayed arrangement. In addition to the water vascular system, there is a system of bloodvessels. Well-defined excretory organs are absent. The sexes are almost always separate. There is usually a striking circuitousness or indirectness in development. The diet is vegetarian (most sea-urchins), or carnivorous (star-fishes), or consists of the organic particles found in sand and mud, the Holothurians in particular practising this worm-like mode of nutrition. Most Echinoderms have to a remarkable extent the power of casting off and regenerating portions of their body. This power is pro- bably one of their means of defence, but they often mutilate themselves as a consequence of unfavourable conditions of life. This self mutilation, or autotomy, seems to be reflex, and not Fig. 98.—Pluteus larva with rudi- ment of adult. — After Johannes Miiller. General Notes on Structure. The Echinoderma, in spite of the numerous fossil representatives, form an exceedingly well-defined group, showing no close relation to any other, and exhibiting certain striking peculiarities. The skeleton is generally well-developed; in Holothurians it consists of isolated spicules, but elsewhere of a series of plates which may be firmly united together, as in most sea-urchins, or may be capable of movement upon one another. Apart from the skeleton proper, lime may appear in almost any of the organs of the body, except heart and gonads. With this deep-seated tendency to form skeletal substance, may perhaps be associated the sluggish habit of the majority, and the absence of definite excretory organs.
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
