422 
MOLLUSCOIDEA. 
able to travel about; a colony of moss-animals capable of locomotion is figured 
on p. 425. These remarkable moving types (6 ristatella') form flattened, 
elliptical colonies, which creep 
along on a kind of flat foot, follow¬ 
ing the direction of the light. 
The question may be raised as to 
how the many separate individuals 
manage to move in the same 
direction. Even if an external 
stimulus, such as light, should 
stimulate the individuals in the 
same way, this seems hardly 
sufficient to account for the 
movement of the colony, without 
some nervous system connecting 
the polyps and co-ordinating the 
movements of the colony. As a 
matter of fact, such a system does 
exist. While each separate animal 
is provided with a nerve-ganglion 
between the oesophagus and the 
posterior opening of the alimentary 
canal (c in the illustration on p. 
420), and with the nerves necessary 
for its own individual organisation, 
the Bryozoan colony, as such, has 
lace-coral, Lepralia (nat. size). 
a special nervous system which is connected with the individual systems, and 
runs from one to another through the apertures by means of which also the body- 
fluids circulate throughout the colony. This colonial nervous system no doubt 
regulates the movement of the stock. 
0rder In contrast to the Phylactolsemata are the Gymnolsemata, those 
Gymnoisemata. Bryozoa in which there is no lid to the mouth, and in which the 
tentacles are arranged in a circle on a disc instead of in the shape of a horseshoe; 
the name given to such forms denoting the naked condition of the mouth. These 
naked-mouthed Bryozoans are far more numerous than those with lids to their 
mouths. Paludicella, which is fully described on pp. 419, 420, is one of the few 
fresh-water forms belonging to this order. Here the crown of tentacles cannot be 
completely protruded, and thus appears, even when most extended, to be surrounded 
by a double collar. A numerous group of this order are the marine Chilostomata, 
or lip-mouthed Bryozoa, of which the sea-mat (Flustra foliacea), common in the 
North Sea, is an example. The magnified cells shown in the illustration represent 
the harder portion of the animals, into which the soft anterior portions can be 
withdrawn. The openings through which the tentacles protrude lie crosswise, 
and each is provided with a lip-like elastic lid. Each individual can thus take 
refuge within its chamber and close the lid. Other genera which, unlike Flustra , 
have no lid, can close the aperture by means of muscles. The colonies of sea-mats 
