38 EVOLUTION OF LIFE, 
composed of a head and a body; sometimes the head and 
body coalesce. The head is furnished with fine hairs ar. 
ranged in different manners, and when these cilia are in 
action they look like wheels. The other end of the body 
terminates in a jointed foot. Both the wheel-organs and 
foot can be drawn within the case in which the body of the 
Rotifer is inclosed. This case resembles that of the Crabs, 
The Rotatoria possess’ the water-vascular system of the 
Worms, as described in Aspidogaster. The group is in- 
termediate in its structure between the Soft Worms, the 
. Annelida, and the Crabs,—-the Rotatoria having been con- 
sidered to belong to each of these groups by different 
naturalists. They represent very naturally that point of 
the tree where the Soft Worms end and the Crabs begin. 
Before leaving the Articulated Worms, the position of the 
Artisca must be noticed. They have been called Tar- 
digrada, from the slowness of their movement; they are 
usually considered as nearly related to the Spiders; others 
have looked upon them as Annelids, while some have con- 
sidered them as the links between the Soft Worms and 
Rotatoria. They are placed, therefore, near these groups, 
without assigning to them a definite position. From the 
difficulty experienced in their classification, the Rotatoria 
and Artisca afford a striking proof of the truth of an evo- 
lution of these worms in some such manner. Having 
called attention briefly to the Soft and Articulated Worms, 
we pass to the last division, the Sac-worms, which includes 
the Bryozoa and Tunicata. 
The Bryozoa resemble living moss, and are found both in 
fresh and salt water. When observed under the microscope, 
this moss is seen to be composed of minute tubes, in which 
the Paludicella, a Bryozoon (Fig. 37), lives. Though this 
creature is small, it is more complex than many of the 
animals we have called attention to. The Paludicella has 
a mouth, gullet, stomach, and intestine, which are entirely 
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