SCOLECIDA. 117 
CHAR PER: ai 
CLASS II SCOLECIDA. 
In the second class of the sub-kingdom Azzuloida are included 
a number of organisms which are, in many cases, very unlike 
one another in external appearance, but which nevertheless 
agree in one or two structural points of importance. The most 
important of these are the possession of a system of water-vas- 
cular vessels, the absence of a vascular system, and the posses- 
sion of a nervous system composed of no more than one or two 
nervous masses or ganglia. The points by which the Scolecida 
are distinguished from the Echinodermata are, the absence of 
calcareous matter in the skin, the absence of any traces of a 
radiate arrangement of their parts, especially of the nervous 
system, the constant absence of any blood-circulatory apparatus, 
and the course of their development. The Scolecéda are often 
vermiform in shape, but many of them exhibit no worm-like 
characters, and one whole order is entirely microscopic. A 
great many of the Sco/ec¢da are internal parasites in other ani- 
mals, and these are often collectively spoken of as Extozoa 
(Gr. extos, within; zo6m, an animal). These parasitic forms 
subsist by: an imbibition of the juices of their host through 
their delicate integument. They have, therefore, no necessity 
for acquiring food for themselves ; and we find, in consequence, 
that many of them are wholly destitute of an alimentary canal, 
and that in all the organs of “relation” are very rudimentary. 
The Scolecida are divided into the following seven groups or 
orders :— 
. Leniada (Tapeworms). 
. Lrematoda (Flukes). 
. Lurbellaria (Ribbon-worms and Planarians). 
. Acanthocephala (Thorn-headed worms). 
. Gordiacea (Hair-worms). 
wmBBeO nN 
