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lated structure as in Retepora, Retihornera, and Fascicula- 
ria ; and the Fenestellidae, which were so numerous in the 
Paleozoic seas, grew also in a reticulated form, and from this 
they have been considered to be allied to Retepora, but they 
entirely differ in every other respect from Retepora, and as 
I have just had the opportunity of observing in Actinosoma 
fenestratum, Young, the so-called denticles are so thin and 
reach so nearly to the centre as to appear like distinct surface 
septae and seem to indicate that the Fenestellidae should 
not be placed with the Bryozoa. 
We said that the Cheilostomata were sometimes cal- 
careous and sometimes horny, and as the Flustra (sea- wrack) 
and Eschara are the best known to casual observers we 
may describe the polypides in these and say that they only 
vary from others in minor details. The polypide is provided 
with tentacles (varying from 8 to 30) which are covered 
with cilia, and from this the Bryozoa were called by Farre* 
the ciliobrachiata. The polypide, including the tentacles, 
when withdrawn, lies in a transparent sheath, which is held 
in its place by sets of muscles from the cell wall, and 
besides these there are the large retractors attached near the 
base of the polypide ; the opercular muscles, which in some 
species, as Myriozoon truncatum, attain great dimensions; 
and in the non-rigid species the parietal muscles. 
The muscles, instead of occurring in solid bundles as they 
do in the higher animals, are in separate threads. Although 
not entering into any details concerning the muscular 
system, it is not because their position is not of considerable 
importance when considering the zoological affinities of the 
various Bryozoa and other lower forms of life; and in 
advocating their relationship with the worms, Schneiderf* 
has pointed out that in the worms their specific and generic 
place can be determined by means of their muscular system, 
in the same way as that of the vertebrates by their bones. 
* “Observations on the minute structure of some of the higher forms 
of Polypi.” Phil. Trans., 1837, pt. 2. 
f Schneider. “ Archiv. fur Micro. Anat.,” tom V., 1869. 
10 o*' 
