262 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



truncation, but at rather regular intervals around its edge and over 

 the tip of tho oral test process. These tost cells are not connected 

 with any nerve fibrils, and they are not in contact with any zooid or 

 any other cells, yet each produces, or causes to bo produced, at the 

 proper place on the surface of the test, the particular bit of sculpturing 

 for which it is responsible. 



Other similar denticles are found over the surface of the test, not 

 in relation to oral processes or the zooids. The cells which form theso 

 scattered denticles are acting in relation, not to the zooids, but to the 

 colon}^ as a whole. It is the "soul" of the colony, and not of any 

 zooid, to which in their activity they are each subservient. 



But we find more remarkable illustration of form control, in the 

 case of the four quadrangular tentacle-like processes at the open end 

 of the colony in Pyrosoma agassizi (fig. 6, pi. 18). These are not con- 

 nected with any particular zooids or groups of zooids. They are 

 composed of the cellulose test material with a few scattered test colls 

 within them. Like the rest of the test, their material is nonliving 

 substance. The test is secreted chiefly, or it may be wholly, by the 

 ectodermal epithelium, the share of the test cells in this function 

 being doubtful. How can these quadrangular test "tentacles", 

 which apparently must elongate by growth at their bases, be so con- 

 trolled that they assume their very definite quadrangular tapering 

 form and lie in their four appointed places at the open end of the 

 colonies, their outer and lateral angles being continued as ridges back 

 over the surface of the colony, well toward its closed end ? 



Assuming for the moment that the formation of the four test 

 "tentacles" is controlled by test cells, we find again an illustration 

 of subservience of isolated colls to the colony as a whole and not to 

 the zooids from which they arose. The "sense of form" (!) which 

 these cells possess has reference not to themselves, nor to the zooids 

 of which they were constituent parts, but to the colony as a whole, 

 although they are isolated cells, lying at an appreciable distance from 

 other cells or tissues, and not connected with any nerve fibrillar. 



Or, choosing tho other possibility and disregarding tho doubtful 

 activity of the test cells, we may say that the test substance composing 

 these tost "tentacles" is secreted by a number of small, discrete 

 areas of ectoderm, tho ectoderm of the several zooids. The charac- 

 teristic features of form of these "tentacles" are superficial and 

 therefore separated by an appreciable distance, tho thickness of the 

 test, say 1-2 cm., from tho secreting cells. How can tho behavior 

 of the test material, aftor it is formed, be so controlled that it assumes 

 tho proper relations of form and position, so that "tentacles" of the 

 characteristic typo and in tho characteristic position, result? The 

 tost material is not alive, yet, at a distance from the cells which 

 secrete it, it molds itself into particular form and takes a definite posi- 



