422 



MOLLUSCOIDEA. 



able to travel about; a colony of moss-animals capable of locomotion is figured 

 on p. 425. These remarkable moving types (Cristatella) 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 

 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. 



Order In contrast to the Phylactolsemata are the Gymnolaemata, 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 withdrawn.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 



LACE -CORAL, Lepralia (nat. size). 



