8 PROF. ALLMAN ON THE MARINE POLYZOA. 



be much more distinctly differentiated into cell-territories. Em- 

 bedded in its substance are oval nuclei, each of which is surrounded 

 by a small area of protoplasm, from which fine filiform off-runners 

 pass out to unite with neighbouring ones. We thus obtain the 

 appearance of a set of stellate cells united by their radiating ex- 

 tensions, and believed by Ehlers to undergo slow changes of form. 

 Ehlers believes these to be of tbe same nature as the stellate cells 

 which Claparede* has observed in the walls of the marine Polyzoa, 

 and to which he has referred the canal-system noticed by Smittf 

 in the body-wall of M&mbrcmpora pilosa. Similar cells have been 

 described by Nitsche in the marine Polyzoa. 



In the completely retracted state of the polypide the appear- 

 ance of a radially striated circular disk, perforated in the centre, 

 may be seen a little within the orifice of invagination stretching 

 across the tentacular sheath in the manner of a diaphragm. In 

 the exserted state of the polypide this appearance is seen to be 

 due to folds in the wall of the tentacular sheath ; and this part of 

 the sheath will then be found to form a transparent, short, cylin- 

 drical neck with longitudinal ridges. 



It will thus be seen that the great interest of Ehlers's memoir 

 consists in its making known to us a type of Polyzoa in which 

 there is expressed a strongly marked dimorphism of the zooids 

 with distinct functions allocated to each of the two forms which 

 thus make up the complete colony. 



* Claparede, " Beitrage zur Anat. und Entwickel. der Seebryozoen," Zeit. f. 

 wissens. Zool. 1871. 



t Smitt, " Om Hafs-Bryozoernas utyeckling," CEfversigt, 1865. 



