Each consists of a prolongation of the syncytial material of the proboscis skin, penetrated by canals and sheathed with a scanty muscular coat.
Until, however, further evidence is forthcoming in support of this syncytial theory of structure, it would be unwise to regard it as established sufficiently to constitute a serviceable working hypothesis; hence, for the time being, we must accept the assertion that the cell represents the ultimate tissue-unit.
In effect (6) it traces the Turbellaria to small two-layered organisms consisting of an outer ciliated epidermis and a central syncytial tissue.
The careful study of the development of one Acoelous form and of certain Rhabdocoels has strengthened this hypothesis by showing that no definite enteron or gut is at first laid down, but that certain embryonic syncytial tracts become digestive tracts, others excretory, others again muscular.
Croup may also be caused by influenza A and B, adenovirus, measles, and respiratory syncytial virus (RSV).