38 BULLETIN OF THE 



alimentary tract iu 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 

 hcemo-lymph system, for in many animals with incomplete vessels, such 

 as Arthropods, Hirudinea, etc., it evidently does so to a certain degree. 

 Moreover the constant motion of the fluids of the body cavity of Eryozoa 

 points to the same thing. It is conceivable that the food in the digest- 

 ive cells might be distributed thi'oughout 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 consimtiing tip, from the youngest functional polypide ; 

 and, secondly, the fact that the tip is separated from that polypide by 

 one or two septte, 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 fi'om 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 commxniication 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 amoeboid. 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 layer 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. 



