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 celom and the 
characteristic water vascular system (hydrocel), 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 ts an ill-defined lacunar system of blood 
vessels, In the hemal vessels, water vessels, and celom, 
there are abundant migratory amebocytes. Well-defined 
excretory organs are absent. Gonads arise on the lining of 
the body cavity, and are radt- 
ately disposed except in Holo- 
thurians. The sexes are almost 
always separate. There ts 
usually a striking circuitous- 
ness or indirectness in develop- 
ment. The larve are almost 
always free-swimming, and 
exhibit a metamorphosis. The 
diet ts vegetarian (most sea- 
urchins), or carnivorous (star- 
Jishes), or consists of the organic 
particles found in sand and 
mud, the. Holothurians in par- 
ticular practising this worm- 
like mode of nutrition. . 
Most Echinoderms have toa ana oh a Aral 
remarkable extent the power After Johannes Miiller. 
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. 
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 
