118 MICROSCOPICAL STUDIES. 



the currents set up by the cilia that clothe the inner or adoral surface 

 of the tentacles. The alimentary canal is of the typical U shape, and 

 is of the most primitive description. 



A very curious point is the absence of any body-cavity (coelome). Its 

 place is taken by gelatinous tissue lying between the alimentary canal 

 and the body- wall. Muscular fibres are present in the stalk, and others 

 control the movements of the tentacles. 



The nervous system is little developed, consisting of a single gang- 

 lion lying beneath the oesophagus, and from which nerves proceed to the 

 tentacles. Sensory cells are found on the outer surface of the tentacles. 



The excretory organs consist of two ciliated tubes opening close to 

 the central ganglion. 



The zooids are hermaphrodite, but it is probable that the reproduc- 

 tive products ripen at different periods. The larvae are Trochospheres 

 tiny swimming spheres characterised by a circlet of cilia around the 

 body anterior to the mouth (preoral) as in the Polychaetes, and the 

 adults themselves retain a majority of the essential features distinctive 

 of this larval form in the persistence of the simple alimentary canal 

 of the latter, of a ciliated ring surrounding the mouth and anus (the 

 tentacles are simply elongated processes of an encircling band), of the 

 two ciliated canals of the larva as nephridia ; the epistome is present in 

 the trochosphere as a tufted eminence, and other minor common 

 characteristics can also be traced. 



The larvae pass a considerable period of their development within 

 the vestibule, which, with the tentacles protectingly overarched, forms 

 an efficient incubatory pouch. 



The higher Bryozoa fall naturally into two divisions according as 

 the habitat is marine or fresh-water. The former are distinguished by 

 the lophophore being circular and are termed Gymnolaemata ; the latter, 

 the Phylactolaemata, by the lophophore being horse-shoe shaped. In 

 all, the anus lies outside of the lophophore, and this is the great 

 distinguishing characteristic. 



Of the fresh-water forms, Lopliopus and Plumatella are typical, and 

 resemble one another closely in all essential features. Plumatella, often 

 found attached to water-weeds in canals and pools, has a creeping 

 chitinoid stolon from which the zooids rise at short intervals. In 

 Loplwpus, the zooids are crowded into dense tufts and enclosed in a 

 gelatinous matrix. In both, the zooids are capable of being wholly 

 retracted within the zooecia or cell-like cuticular envelope, chiefly by 

 means of a powerful strand of muscular fibres (retractor muscle, r.) 

 attached at one end to the anterior portion of the oesophagus and at 

 the other to the lower inner surface of the resistent stolon-tube. 

 Besides this, the zooid can be partially retracted by a set of small 



