290 



PHYLUM ECHINODERMA 



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

 pouches grow out to form the usually spacious ccelom and the 

 characteristic water vascular syste?n {hydrocoel), 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 hcemal 

 vessels, water vessels, and 

 coelom, there are abundant 

 migratory amoebocytes. 

 Well-defined excretory 

 organs are absent. Gonads 

 arise on the lining of the 

 body cavity, and are radi- 

 ately disposed except in 

 Holothurians . The sexes 

 are almost always separate. 

 There is usually a striking 

 circuitousness or indirectness 

 in development. The larvce 

 are almost always free- 

 swimming, and exhibit a 

 metamorphosis. The diet 

 is vegetarian {most sea-urchins), or carnivorous {starfishes), 

 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 probably one of their means of defence, but they often 

 mutilate themselves as a consequence of unfavourable condi- 

 tions of life. This self -mutilation, or autotomy, seems to be 

 reflex, and not voluntary. 



'"'>.. 



-Pluteus larva of Ophiur- 

 oid, with rudiment of adult. — 

 After J ohannes Miiller. 



Fig. 157. 



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 



