MUSEUM OF COMPARATIVE ZOOLOGY. 37 



cells of coelomic epithelium. Eisig ('87, p. 752) has already clearly ex- 

 pressed how, iu view of the iiiauy cases of high excretory activity of 

 peritoueal and blood cells demonstrated by him, " kiiuftighiu hei der 

 Beurtheilung gewisser Zelleneinschliisse erst geuau festzustellen sein wird, 

 ob man est mit voii aussen aufgenommenen (gefressenen), oder aber mit 

 von der Zelle ausgeschiedenen Producten zu thim habe." 



A criterion forjudging this matter may be found, in the first place, I 

 believe, in this : that the products of excretion increase with the activi- 

 ties of the cells, and are thrown out, usually in the shape of concrements, 

 either from the cell or with the cell into the coelom ; whereas bodies 

 taken in from without for digestion decrease with the activities of the 

 region. In the second place, vacuoles are less characteristic of excretory 

 "tissue than of imbibitory. But vacuoles are the important feature of the 

 reticulated cells in Paludicella, and the highly refractive bodies are less 

 constant phenomena. As for the latter, they are not found in the later 

 stages, nor in the earliest. Moreover, these bodies differ from excretion 

 concrements in this, that they are always transparent, often almost indis- 

 cernible in the vacuole, except by their higher refractiveness, and there 

 is no sharp demarcation between cases of vacuoles fiilled by such bodies 

 and those the contents of which are less highly refractive. The degree 

 of refractiveness is variable, at one end of the series grading off into the 

 undifferentiated fluid of the vacuole. What significance is to be assigned 

 to these highly refractive bodies in the vacuoles % There are two reasons 

 why I do not believe that they represent solid food particles devoured 

 as such by the mesodermal cells. First, I do not find such highly 

 refractive bodies lying loose in the body cavity before the stage at which 

 they first appear in the cells ; and, secondly, one can find all gradations 

 between less highly refractive vacuoles and highly refractive ones (which 

 I have assumed to be entirely filled by one highly refractive body), and 

 between the latter and vacuoles containing a small body surrounded by 

 a broad, clear area. I believe, therefore, that the vacuoles are i-ather 

 cavities filled with chemically different nutritive fluids, which are acted 

 upon difterently by the reagent. 



I have assumed that the contents of the vacuoles represent matei'ial 

 taken up from the body cavity, because it seemed most reasonable to 

 look there for the som-ce of their supply. The ectoderm is covered on 

 its outer surface by an apparently continuous cuticula, so that food 

 cannot be gained from the outside world directly. It is, moreover, not 

 unreasonable to suppose that some of the products of digestion elabo- 

 rated by the adult polypides of the colony pass through the wall of the 



