38 BULLETIN OF THE 
alimentary tract in solution, and thus into the body cavity, from which 
they may be taken up by the mesodermal cells at the growing part 
of the body wall. Nor is there anything unreasonable in insisting that 
the body cavity functions, in these animals without blood-vessels, as a 
heemo-lymph system, for in many animals with incomplete vessels, such 
as Arthropods, Hirudinea, ete., it evidently does so to a certain degree. 
Moreover the constant motion of the fluids of the body cavity of Bryozoa 
points to the same thing. It is conceivable that the food in the digest- 
ive cells might be distributed throaghout the body wall without passing 
into the body cavity, since all parts of the body wall are continuous 
with the digestive epithelia of the polypides of the colony. Two consid- 
erations make it improbable that the cells of the tip gain their nutri- 
tion in this manner from the digestive cells of the youngest functional 
polypide: first, the considerable distance of the rapidly growing, and 
hence rapidly consuming tip, from the youngest functional polypide ; 
and, secondly, the fact that the tip is separated from that polypide by 
one or two septa, whose central cells are highly metamorphosed, and 
apparently cuticularized, thus serving to break the continuity of the 
cell wall. An objection to the assumption that the mesodermal cells of 
the tip derive their nourishment from the products of digestion which 
have been elaborated by the alimentary tract of the youngest polypides 
and passed into the body cavity, might be based on the fact that the 
communication plates are always fully formed between the bud and 
the next older polypide before the older polypide has become functional. 
If the communication plate were a closed septum, this would be a fatal 
objection. But it is not closed to fluids carrying food in solution, The 
very persistence of an opening indicates that it has a function, and favors 
the hypothesis here presented. 
Positive evidence for the conclusion that the reticulated mesodermal 
cells take up food material from the body cavity is derived from the 
fact that these cells often show evidences of being amosboid. Thus they 
are sometimes found with pseudopodia-like prolongations of the cell body 
(Figs. 54 and 59). <A large percentage of all reticulated cells of this 
stage show similar appearances. Although they here seem to keep their 
places in the mesodermal epithelium, their movements being confined 
to their free surfaces, the cells derived from the homologous Jayer in 
marine Bryozoa are migratory. Therefore these may be considered as 
morphological equivalents of migratory cells, which have come to remain 
in or have never departed from the mesodermal layer, although possess- 
ing some of the characters of these notoriously trophic elements. 
