ECHINODERMATA. 103 
SUB-KINGDOM III. ANNULOIDA. 
CHAPTER X. 
ECHINODERMATA. 
Tue third primary division of the animal kingdom is known by 
the name of Aznuloida, and includes two groups of organisms 
which are extremely unlike one another in appearance, and are 
termed respectively the Echinodermata and the Scolecida. In 
the former we have the sea-urchins, star-fishes, and their allies, 
formerly classed in the old sub-kingdom /Aadzaza, in the latter 
are a number of internal parasites, with some minute aquatic 
creatures, all formerly referred elsewhere. Different as are 
these two groups in appearance and habits, they are neverthe- 
less united by the following peculiarities:* They possess a 
distinct alimentary canal, usually communicating with the outer 
world by two apertures (a mouth and a vent), but in any case 
completely shut off from the general cavity of the body (fig. 43, 2 
a, b,c). In all there ts a distinct nervous system, and in all 
there ts a peculiar system of canals termed the “water-vascular” 
or ‘“aguiferous” vessels, which usually communicate with the 
extertor of the body. It should be mentioned that many natural- 
ists dissent from this grouping together of the Echinodermata 
and Scolecida into a single sub-kingdom, Aumulotda. Many 
other arrangements have been proposed, most of which present 
some special advantages and some disadvantages. Inthe mean- 
while, in the confessedly uncertain state of this department of 
Natural History, it has been thought well to adhere to the 
arrangement proposed by Professor Huxley—an arrangement 
with many obvious drawbacks, and at best but provisional. 
* Some of the internal parasites of this sub-kingdom have no alimentary canal at 
all; but this does not affect the value of the above definition. 
