240 



PHYLUM ECHINODERMA. 



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 ccelotn 

 and the characteristic water vascular system (hydroccel], 

 which may have loco motor or respiratory functiojis 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 lacunar system of 

 blood vessels. In the hcemal vessels, water vessels, and ccelom, 

 there are abundant migratory amoebocytes. Well-defined 

 excretory organs are absent. Gonads arise on the lining of 

 the body cavity, and are radiately disposed except in Holo- 

 thurians. The sexes are almost always separate. There 



is usually a striking cir- 

 cuitousness or indirectness in 

 development. The larvce are 

 a hnost a Iways free-sivim m ing, 

 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 particular practising this 

 worm-like mode of nutrition. 

 Most Echinoderms have to 

 a remarkable extent the power 

 of casting off and regenerat- 

 ing portions of their body. 

 This poiver 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. 



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 



FIG. 114. Pluteus larva of Ophiur- 

 oid, with rudiment of adult. - 

 After Johannes Miiller. 



