GENERAL NOTES ON STRUCTURE. 



253 



tissues (mesenchyme}, and in consequence there is frequently a 

 very complete skeleton. From the primitive gut of the larva, 

 pouches grow out to form the usually spacious ccelom and the 

 characteristic water vascular system (hydroccet), 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 an ill-defined lacunar system of blood 

 vessels. In the hczmal vessels, water vessels, and ccelom, 

 there are abundant migratory amcebocytes. Well-defined 

 excretory organs are absent. Gonads arise on the lining of 

 the body cavity, and are radi- 

 ately disposed except in Holo- 

 thurians. The sexes are almost 

 always separate. There is 

 usually a striking circuitous- 

 ness or indirectness in develop- 

 ment. The larvce are almost 

 always free-swimming, and 

 exhibit a metamorphosis. 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 par- 

 ticular practising this worm- 

 like mode of nutrition. 



*/r * T- z. j i, * FIG. 131. Pluteus larva of Ophmr- 



Most Echinoderms have to a ^ >,., v, m ^ metnt . ^ .23, _ 



remarkable extent the power 

 of casting off and regenerating 

 portions of their body. This power is probably 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. 





old, with rudiment 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 



