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 voluntary. 



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. Except in Holothurians, where the calcareous plates 

 are diffusely scattered, the parts of the skeleton show much regularity 



